So it IS a protocol. Which means it could be used on other operating systems. Just not Apple.
Well, yes and no. The problem is that quite often, people start writing code before thinking through (thoroughly) the data model and portability of what they are doing. Things developed on WIndows tend to fall into that trap quite regularly.
The problem is that ASCOM is really tightly tied into a bunch of Windows tooling. While there is a wire protocol component, the only way its really getting rendered is through Windows proprietary technology. So, one COULD do an independent implementation, but it would be much more work than just doing it the 'right way' from scratch. There would also be no guarantee that future versions of the 'mother' project wouldn't break all your hard work.
The alternative, INDI can be run on anything, ASCOM, much to many folks annoyance will probably always be just a Windows tool.