Good point! Though that does not, of course, negate that there were only so many TRS-80s out there to which to attach drives, and there seemed to be a lot more drives than TRS-80s and Apple IIs.
It was also, I think, a lot easier for third-party companies to supply drives for the TRS-80:
- The TRS-80 (as far as I know) used the standard SA-400 style drive and electronics, rather than Apple’s more heavily customized drive.
- Anybody with an expansion unit (or, later, a Model III) already had the (computer-side) drive controller, whereas Apple II users needed to buy a controller from Apple or vendors needed to clone and manufacture it.
(Certainly for the Radio Shack Colour Computer the interface was standard to the point that, though I bought the drive controller cartridge from Radio Shack, I never bought a drive from them. My drives were just generic parts that came out of an advert in the back of Byte magazine and, IIRC, I powered them with an IBM PC power supply.)
The first advertisement in the first issue of 80 Microcomputing (January, 1980) was for third-party drives from Percom. But even as early as February 1979 (only 6 months after Radio Shack released their drive) there was an advertisement in the back of Byte for TRS-80 disk drives from a third-party. (This doesn’t mean they were actually available—in those days of multi-month lead times advertisements were frequently ordered before the product was finished—but it does seem to indicate that someone at least thought they wouldn’t be too hard to make.)