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It's an open question whether desktops would've kept using 5.25" until the end of the floppy era, but laptops meant something smaller was going to be introduced; that much was essentially predetermined. The contingent historical fact was the adoption of the particular 3.5" format we all remember, in preference to the many others that were contenders at the time.

I was reading this Wikipedia page just now and came across this:

"In the early 1980s, a number of manufacturers introduced smaller floppy drives and media in various formats. A consortium of 21 companies eventually settled on a ​3 1⁄2-inch floppy disk (actually 90 mm wide) a.k.a. Micro diskette, Micro disk, or Micro floppy, similar to a Sony design but improved to support both single-sided and double-sided media, with formatted capacities generally of 360 KB and 720 KB respectively."

So the way Wikipedia puts it, sounds like the decision was basically made by committee. Twenty-one companies got together, carried out a sober evaluation of all the contenders based on technical merit, manufacturing cost, which influential members already had a large investment in what, etc, then issued a verdict and so it was done.

My understanding had been a bit different. As I understood it from e.g. here the big breakthrough for the 90 mm format that ended up winning, was getting into the Macintosh, for which Apple helped Sony debug the drives (their own Twiggy drives developed for the Lisa, never having become reliable enough). I assumed this was the reason they started being used in PC compatible laptops, which settled the matter.

If that version of the history is correct, the outcome was determined not so much by a grand deliberate decision from all interested parties, as by a few particular events, decisions made by a handful of individuals who were trying to solve their own short-term problems; a historical accident, chaos at work in the technical sense of the word.

Which version is accurate?

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    Speaking as a user of 8", then 5.25", and lastly 3.5" floppies, I'd suggest "comes in a rigid cover" was a significant factor. – another-dave Jan 18 at 23:37
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    yup what a blast from the past @another-dave, i used to use 5.25 ones in school to load dos and remember having to baby them on strict instructions from the teacher not to damage them. then 3.5 rigids came into vogue and i would fling them across the schoolyard to give a class project to someone else – Karan Harsh Wardhan Jan 19 at 6:34
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    A real LOL answer. Remember the days of pocket protectors and slide rules. Didn't think so. A 3.5" floppy will fit in a dress shirt breast pocket. It was a practical reason. Ever heard of sneakernet – Guy Coder Jan 19 at 16:56
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    I recall Steve Jobs being interviewed and related an engineer trying to sway Jobs into using the 3.5". When Steve asked, "Give me a really good reason...". The engineer took it and placed it in his shirt pocket. Steve then said "Ok, go with that!". – jwzumwalt Jan 24 at 1:45
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    I don't believe this question should be marked as duplicate. This is a question specific to the 3.5" disk drive. The other question is specific to the capacity, not the physical drive itself. – Jason Hutchinson Jan 28 at 21:00
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So the way Wikipedia puts it, sounds like the decision was basically made by committee.

And that's what it was - and what made it succeed. A standardized disk format with a drive interface compatible with existing controllers.

As I understood it, the big breakthrough for the 90 mm format that ended up winning, was getting into the Macintosh [...]

Not really. For one, Apple used a Sony drive from before the standardization mentioned. While the mechanical and media part was the same, the drive did differ in its interface and operation, thus requiring dedicated controllers.

I assumed this was the reason they started being used in PC compatible laptops, which settled the matter.

More or less. There were eventually 3 major steps and some in-between development marking this process, with IBM's use of 3.5 inch drives in their PS/2 line as the final milestone.

  • In 1980, Sony developed the 3.5 inch format. Only a few computers like the HP-160 or Sony's SBC-70 used that drive.

  • In 1982, the 3.5 drive as we know it got defined by a joint committee. The approach followed was to use Sony's mechanical and media design, but use an interface compatible (*1) to the existing Shugart standard for 8 and 5.25 inch drives. Only the connector was turned from PCB into a pin header for size reduction. This had the advantage that all needed was a new cable to operate a 3.5 drive on existing 5.25/8 inch controllers.

  • 1983 brought the first drives to this standard, offering 360 KiB (single sided) or 720 KiB (double sided) when operated with standard MFM controllers. Beside many small machines, a first batch of drives for MSX computers opened a door in the consumer market.

  • Eventually the first PC(-ish) computer to use 3.5 inch drives was the Apricot PC in 1983.

  • 1983/84 was when Apple adopted a drive, based on the Sony design, but incompatible with the standard, for their Mac. The deviation was to increase capacity and reliability at the same time. While it worked great, its impact on the floppy marked could be ignored as Macs didn't gain much of a market share and the drive itself wasn't sold to other manufacturers.

  • 1985 saw Atari and Commodore adapting standard-compatible drives for their new 16-bit machines. Around the same time, 3.5 inch also established itself as the standard format for MSX computers in Japan and Europe (*2). In combination, these home machines created a huge user base lowering cost of drives and media at and below existing 5.25 inch drives.

  • 1987 saw IBM introduce their PS/2 line with 1440 KiB 3.5 inch drives (doubled as HD) as standard. Even though PS/2 sales were, lets say, less than optimal, PC manufacturers rushed to embrace the 'new' format to show their advancement.

Shortly thereafter (1988 or 1989, depending on source) sales of 3.5 inch drives surpassed 5.25 sales ... and the rest is history.

  • oh, and then there was ED (2880 KiB) in 1990, but that only caught on in Japan, despite IBM offering some PS/2 with ED drives.

*1 - Here hides the true secret, compatibility. It already worked well, enabling the move from 8 to 5.25 inch. At the time the 3.5 was designed, many new drive variations between 2 and 4 inch were developed (IBM's Demidrive being a good example). Most had their own 'way' improved interfaces. None got a large distribution - except those using a Shugart compatible interface. The 3 inch is a great example that worked.

*2 - MSX2 made the 3.5 inch drive standard. 5.25 were still supported, but all manufacturers switched to 3.5 for their new machines.

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    @T.E.D. Well, you might be onto something. The internet is of course always right. BTW, I found a site explaining in detail why the earth is flat. I bet Wiki is following soon. :)) But serious, one of the important parts when citing wiki is always double checking information found - and as shown, Motorola did provide a different view. So who's right, genuine documentation of 1983 or a Wiki entry made 30 years later by 'some people on the internet'? Beside, the time we fought to upscale our machines by attributing it as 32 bit or whatsoever is long past. Time to get serious again, Isn't it? – Raffzahn Jan 18 at 18:53
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    Despite the committees and what not, there's no mention that the 3.5 design was simply, fundamentally, better. More durable, convenient, smaller, faster, higher capacity. Eventually going to the 2.8M models that the NeXTStation (among others) supported, but by then it was too little too late. – Will Hartung Jan 18 at 19:28
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    Nobody back in the day called the 68000 a 32-bit processor. It was a 16-bit processor with a 24-bit address space, just as the 8080/6502 was an 8-bit processor with a 16-bit address space. The 68000 had 16 bit registers and could do native 16-bit math in one instruction, and that was that. And that was big at the time! – Harper - Reinstate Monica Jan 18 at 22:58
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    @Harper -- no, even the lowly 68000 had 32-bit address and data registers, and I'm 99.999% sure it could do 32-bit addition and subtraction in a single opcode. I was a high school student with an Amiga in 1987, and nobody I knew would have EVER called the 68000 a "16-bit microprocessor" back then (8/16/32-bit, MAYBE... but psychologically, it was unambiguously 32-bit). – Bitbang3r Jan 19 at 2:31
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    The mechanical robustness of the 3.5" diskette was also important in displacing the 5.25". It also had a write-protect slider, whereas the 5.25" involved sticky tabs and and with the 8" floppies, you had to cut a slot in the jacket to write-protect them! – grahamj42 Jan 19 at 10:16
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This is covered in one of the major Mac history works, although I can't recall specifically which one.

When Jobs was putting together his supplier list the 3.5 had been standardized, as Raff notes, but you still had lots of companies pushing their own formats. Machines with all of these could be found on the market.

Jobs went to Japan to visit with the various manufacturers to see where they were, I don't recall anything suggesting he had made up his mind on the format (other than "no 5.25" anyway).

The account notes that in some cases he would be presented with mock ups, and in one case a block of material that was indicative of the size and shape of the proposed device. Apparently he savaged them in these situations, with the book joking that they went away to commit hari-kari after these meetings.

Only Sony had an actual production-quality drive ready to go at the production numbers he demanded. His numbers proved overly optimistic, but the rest is history.

It seems the history is similar to USB in many ways. USB was going to happen sooner or later, but the iMac certainly helped jump-start the process.

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The 3.5" floppy drive was first introduced to the market in 1983 with a single sided version with a 360K capacity. The following year double sided disks were introduced that doubled the capacity to 720K. Eventually the capacity was increased to 1.44M, and even a short lived 2.88M version. Even though these drives were available, it took a long time for them to be adopted as the standard.

The 5.25" floppy, which displaced the earlier 8" drives became the de facto standard for over a decade. Most major software companies such as Microsoft were shipping their software on the 5.25" floppy long after the 3.5" drives were available. Most IBMs, and IBM compatibles did not have a 3.5" drive until the early 1990s, and at that time, companies wanted to maintain backward compatibility with existing hardware. Since most software was still on 5.25", there was not much a need to have both drives, since a floppy drive at that time was an expensive option. Companies began the shift to the 3.5" floppy around 1990. They were eventually phased out by the mid 90s around the time the Pentium was released.

One of the first 3.5" floppies I owned was the game Rampage which actually shipped with the game on a 5.25" floppy and a 3.5" floppy. In fact MS-DOS 5.0 which was released in 1991 shipped on a 5.25" floppy!

Sure, Apple had a 3.5" floppy many years before. But that was not what really forced the change. Apple floppies had their own proprietary file format which were not compatible with a PC. They also had a very small market segment compared to the PC. What really helped drive it is the form factor of the PC changed. Original IBMs had a full size AT board, which was massive. In 1995, the ATX form factor was released, which was significantly smaller. By this time the 5.25" drive has been abandoned for the 3.5" drives because they took up significantly less space. The CD-ROM was also becoming a popular option, and was installed in the drive bay made originally designed for the 5.25" floppy. The 5.25" floppy was already in decline for a few years before, but the CD-ROM effectively killed it.

1

I'm going to answer the question from a practical user perspective, having been an active microcomputer user through this transition.

5.25" floppies have a few disadvantages:

  • Limited capacity. Yes, higher-density versions of these disks could store up to a megabyte or more, but were expensive and rare. 3.5" floppies stored 320-400K on SSDD media all the way up to 1.44 MB on DSHD media (and 2.88 MB on the never-very-popular ultra-high density version which I never used).
  • Fragility. 5.25" floppies were easy to damage because they were flexible. 3.5" floppies had a hard shell so you could throw a disk into a pocket or even mail a disk in an envelope without any great danger.
  • Jackets. Those diskette jackets are a nuisance. You can lose them and misplace them, and have to do something with them while your disk is in the drive. 3.5" disks have a built-in retractable metal shutter that provides superior protection plus the advantage of being able to be forgotten about.
  • Write-protection notches. 5.25" floppies have a notch that has to be covered by an adhesive strip to protect the media from being written to. 3.5" floppies have a plastic switch that can be toggled by the user very easily (no running out of strips) and can be untoggled just as easily (the strip could be removed from a 5.25" disk, but rarely was adhesive enough to be reused afterward).

I think the other reason 3.5" disks took off so much is their size. Computers like the Macintosh, Atari ST series and Amiga 1000 and 500 had built-in floppy drives and yet were fairly small machines. A 3.5" floppy drive stored more and took less physical space. This was less important on desktop PCs, but even there was a help as cases could get smaller or the space used for other devices.

  • Capacity is not related to media size ... if at all, it's the other way around with 5.25 being able to store (size relative) more on the same media type. – Raffzahn Jan 25 at 20:45
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    @Raffzahn Of course, but I'm talking about the practical capacity, not the theoretical capacity. Standard densities of 3.5" disks store more than common densities of 5.25", despite being smaller-sized. – Jim MacKenzie Jan 25 at 21:09
  • Just because the corresponding 5.25s never made big inroad. Well, except maybe Bernoullis with 10 Mib (and later 20 MiB) at a time when 3.5's where stuck at the same 720 as 5.25 did :)) Yes, Bernoullis are floppy disks. – Raffzahn Jan 25 at 21:13
  • @Raffzahn Users don't care about what made inroads. They care about what is available to them. DSDD 3.5" stores more than double what DSDD 5.25" stores. DSHD 3.5" stores about 1/3 more than DSHD 5.25". Bernoullis were never mainstream. Of course bigger media can theoretically store more than smaller media - just use the same density, but that was never done for various reasons ,and users (except for you and fans of tech) don't care why. Harsh to downvote me for this. – Jim MacKenzie Jan 25 at 21:17

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