- Some of the hardware reflects 1980's designs. For example, it uses a CPU that was used back then and is still available. (Edit: or rather a variation of the CPU.)
- Some of the software is from the 1980's. That requires rights holders who are willing to license that software.
Those two criteria alone would limit the systems this type of project could reimagine. Then you have to consider:
- The people behind this project. Creating your dream computer in hardware takes skill and determination. The people following through decided to use the Commoder 64 as their model.
- A large part of the rationale is that it is "easy" and lacks weirdness. Many of the people interested in retro computers are looking for something more engaging than games. They want hardware that is simple enough for them to understand and develop software for at a low level.
Yet I suspect that the biggest reason why the C=64 was reimagined has to do with the community around it today. For whatever reason, Commodore computers have attracted a following of hardware and software tinkerers (this goes for the Amiga as well). I haven't seen quite the same thing to a similar degree of dedication on other platforms, aside from some particularly dedicated 68k Macintosh hardware presevationists and 8-bit Atari software developers.