... Apple could have ruled the world with the Macintosh (as in, Windows/IBM Compatibles wouldn't have had a 90% or whatever market share) but Steve Jobs was just too hard headed and wouldn't let anyone, except maybe Adobe, develop for it. That they wanted to control all the software and have it go through them, and since Microsoft didn't exert any such control, plus they were available on everything, they "won" and Apple floundered for years.
Why did Microsoft take the lead
So, you have to understand a bit out how computing was back then. It's very hard for us to understand today, as we have many computers in our homes, tables, and cells. Back then a computer was not something that a person would have. It was something that a business would have, and maybe just one or two of total, for a huge business.
Microsoft and Apple went after two very different markets at the start. MS went after the business man that wanted to work from home. You could have a MS-DOS PC that had some word processing and spreadsheet app, in your house.
Apple, on the other hand went after hardware. Again this is tricky to understand today. But essentially MS would let anyone that wanted to build hardware and run their OS. Apple on the other hand wanted to make sure that the OS only ran on "their" hardware.
So while apple was focused on pushing their "hardware" Microsoft focused instead on pushing their "software". The push hardware model was certainly "the way things were done" Tandy, Atari, etc, had showed some success in this area, while at the same time, software was thought to be risky.
However Microsoft's approach to pushing software worked out. Other companies (like IBM and Packard-Bell most notable at the start) already had established hardware channels and were able to "retro fit" MS DOS onto existing hardware cheap and easy. A small boost, that Microsoft was able to turn into a big win.
Steve Jobs was just too hard headed..
Possibly, I don't know him personally, but he has a reputation for being stubborn and demanding. Though, he never, at any time that I can recall, outright blocked app development, or even tried to. The app store, as it is today, didn't exist, and it's mostly market share that lead companies to decide to write code or not for apple. That said, he did spear head a decision process that would come back and lend a little truth to this argument.
Few people wrote applications
This is true. Mostly for market share reasons as noted above, but for one other, really big reason. No one wanted to.
As much as we developers go where the money is, we also have a strong ability to influence what platforms we develop for. Microsoft had a massive list of "compatible languages". Back then there was BASIC which was good for hobbies and small programs, but bad for complex programs (though it was used anyway). Assembly, which no one liked but we all could use (a lot of programming back then was just wrapping assembly calls in something else), C/C++, Borland C (C but a little different) Fortran, Delphi, etc. etc. etc. There were libraries and tool chains to help you out, and just a bunch of choices.
On the Apple side of the fence there, Apple decided, early on, to focus on a single good, solid tool chain, instead of a hoge-poge of middling tools. There was more then one language, there was more then one tool chain, but when it all came down to it, there were just more restrictions and fewer choices to work from.
What essentially happened is that "it was no fun", so fewer developers would write code for it. The developers that could or would charged more, and thus an application had to be larger to to make money and support development.
At the same time Apple had carved out a market sector and couldn't really break into new areas. They dominated in Education and Graphic Work, and, in those areas, they had plenty of companies willing to write code for them. The products they made were successful enough they could pay the developers more, so the developers were willing to have "less fun".
On the Microsoft side of things, there were "tons" of developers willing to work for little or no money. Their platform was "fun" enough and had enough choices, that applications, not all of them good, flourished. There were tons of crap applications that barely worked, or didn't work at all.
As an example, lets say 1 in 100 applications for DOS and windows were "good" applications, and 1 in 5 applications for Apple OS were "good". Windows/DOS had thousands of applications, Apple only had 500. Yes apple had the "it works" thing going for them. But if you wanted to do a thing you maybe only had a choice or two to choose from. In Microsoft land you could have 10s or hundreds to choose from, even if a lot of them sucked.
This spiraled a bit, and it looked like Apple would fail, or at least "stay small".
Did Apple originally not let anyone develop for the Macintosh or is this a myth based on some other factor like a lag time in getting developer tools out there?
Neither, their developer tools were out from day one, and they were very open with letting applications on their platform back then. It just wasn't fun, they failed to attract developers (companies or people), so their application pool suffered. MS on the other hand successfully attracted tons of developers. This made dev costs for an Apple app go up and a DOS app go down. So much that you would only attempt to write code for an apple product if it was a "sure thing" and if the potential market was large enough.
There was, back then, never any direct resistance to making an application for Apple OS. It just wasn't fun or profitable to do so. Anyone could do it, and Apple even had some programs to help with costs, but the tools and decisions involved, just didn't appeal to most.