Why did Sharp choose 5.25" over 3.5"?
Why something has 'not' been done will seldom get an easy answer. Often it might be a variation of 'why should they'.
When the X68000 was released in 1987, 3.5" drives were well established and used in several other 16 bit machines, such as the Amiga and Atari ST.
Well established? Maybe, but still neither mainstream usage and even less majority. In 1987 it may have been simply too early for 3.5 inch for a consumer class device. 3.5 was only slowly taking up Speed. The mainstream timeline looks a bit like this:
- Apple had introduced their (very expensive) drive in 1984,
- Amiga and Atari came in 1985,
- IBM only added the first 3.5 in 1986 to the niche PC Convertible ,
- PS/2 started in 1987 to make the 1.44 HD a mainstay,
but it still took the better part of a decade to make 3.5 the standard drive and 5.25 an optional feature for PCs. In 1987 the overwhelming majority of machines and drives sold were 5.25. Going 3.5 as primary (and only) drive was in 1987 still rather unusual and quite avantgarde.
Next, price is crucial for consumer class devices like the X68000. It was developed and marketed by the TV division in Yaita as a follow up to their X1. 5.25 drives were still lower priced than 3.5, an important fact for consumer devices. Not to mention that 3.5 diskettes were more than double the price than same capacity 5.25.
The major competition to X1 and X68000, all the MSX machines, moved at that time slowly from 5.25 to 3.5. Interestingly way faster in Europe than in Japan or other parts of the world.
Which brings the compatibility issue. The previous X1 did as well use 5.25 drives (if equipped with floppies). Similar all MZ machines (*1). For all users upgrading from an X1, or moving over from an MZ, a 5.25 is crucial to continue using their data (*2). Heck, even the only MSX Sharp the Brazilian HB-8000, introduced just short before, did.
Bottom line: For a mass market device intended to upgrade in a price sensitive market it was a sensible decision stay with compatible technology.
*1 -The MZ series in contrast was developed by the computer division in Yamatokoriyama.
*2 - This is still before build in hard disks and USB sticks. Floppies were not only used for data transfer and maybe backup, but the very sole storage, used every day over and over. Thus everyone had a considerable stack of word/data diskettes, usually outnumbering program diskettes by far. Unless it was a strict gaming only setup, but even then many games need save disks as well. Saving onto a program disk was a great way to loose both, saves and the game.