I think it's mostly a patent issue. They still hold quite a few. The same was / is true of the NES. Some of those patents have expired, some have not. It's doubtful that Nintendo will really go to much trouble chasing down people that make clone hardware at this point. I mean, they don't support the system anymore, so while Nintendo could go after these guys for making clones, they probably won't. I suspect many of the patents are still active on the Saturn.
Also, one has to think, with all the moving parts of the Saturn / PSX (and later) consoles, there's more of a cost involved. Whereas the previous clones are pretty much all solid-state, SoC designs, you get more cost overheads with the Saturn. Most of these companies are kind of chipping away at the fringes, and keeping production costs down is huge. The more engineering involved, the less likely they produce a console.