A gazillion months later just because I want to chime in my 2c -- I definitely agree with anon here. Part of the problem is that there's tons of small unique canons that I would love to explore in a non-panfandom setting (the VFD in A Series of Unfortunate Events! The Half-Made World! Galavant-esque fantasy satire! a Fast and the Furious GPSL!), but the odds of getting together a cohort of good like-minded interested players is, well, tough. So wider umbrellas like superheroes/paranormal tend to be the most popular game premises.
nicepants, is there an itch in particular that you want but haven't gotten scratched? I've seen some other stuff -- e.g. zombies, spies, Final Fantasy -- do the rounds sometimes, so there's definitely room for other things that aren't panfan/HP/superheroes/paranormal/celeb. It's just more rare, as you've noticed.