It's hard to work in a genre that isn't well established because you have to stop and think at every step and sometimes do huge amounts of research rather than just relying on past works, so it makes sense that people will either stick to whatever few themes are proven to work, or try to port them over from other genres as best as they can.
The best example I have is from Starcraft mapping where nearly all RPG custom maps (it's very moddable ok) try to be fantasy despite the overwhelming odds they have to face (scifi-styled setting, creatures, sounds and everything) even if it involves renaming what's obviously a space cruiser with laser weapons to "Acid-Spitting Hydra Level 2" and a guy with a rifle to "Elven Archer".
There's a few good scifi rpgs but not many because being the first is fucking hard.
There's nothing much you can do about it other than knuckling down and doing the research, whether it's for completely new shit or good analogues to what works in fantasy/other genres.
Yeah, you are right. It really comes down to doing research. But that's the fun part, for me at least. For my game, I have already done a huge amount of research (being interested in space colonization) before I even conceived of the game idea, so that definitely helps out a lot.