Why are fantasy RPGs more popular than science fiction?
As has been mentioned above, I think the range of technology and having to explain more of how things work is the big factor.
In fantasy you have magic, but explaining how that works is often explained in a class-specific "spellcasting" section. It's all self-contained in classes that want to opt in to how things are different. Everything else operates normally and matches both the fiction and history.
With science fiction you need to explain so much more. Off the top of my head you need to know:
- Projectile weapons or energy?
- FTL or not?
- If FTL, then warp engines or jump drives or hyperspace gates or wormholes? And is it just travel or is there FTL communication?
- If no FTL, then generational ships, cryofreeze, or are people limited to the solar system?
- Artificial gravity?
- Artificial intelligence?
- Genetic modification?
- Brain implants?
- ESP/ psionics?
- Time Travel?
- Teraforming?
I ran a game of FATE Eclipse Phase a while back and there was this huge learning curve as that's a much harder science fiction. No FTL, no scanning for life signs, no artificial gravity, and the like.
Artificial gravity is a big one. It's so common in television, but that's for budgetary reasons. But it'd really be super hard to do without massive spinning ships. Something like
Firefly, which was pretty low tech, probably should have been a zero-g ship.
Licenced settings tend to be the easiest. You know what's possible in
Star Trek. But that's subject to the tropes of that franchise. And not everyone likes those settings.
At the end of the day, players are the problem here. Every DM here knows the pains of getting players to read the damn handouts and world lore. Getting them to learn of the world. It never happens. But that's okay, as you can just start in a small village and slowly expand out to the rest of the universe.
That's harder with sci-fi. You want to begin with the ship. You can't start in a small space station solving local problems. The narrative equivalent is starting on the small mining vessel. But then you need to explain all this tech...