The RPG market is dominated by fantasy (with horror coming in second). Why have most attempts at creating a science fiction RPG failed (commercially or artistically), and what would a hypothetical SFRPG need to catch on the way fantasy has?
I think there are two directions that SF games can go, and the main reason that they haven't take off like fantasy games.
On one hand, you have the hard sf games. On the other, you have space opera.
Both have their fans, and both have their strong and weak points. Since you can't merge the two, there is the big problem.
Hard SF games are ones where the science is paramount. Tech is the big thing, what makes the game go and be fun for those who play it.
For space opera, it's the fantasy, to borrow the term. The big, sweeping epics that have you moving from one point in the galaxy to another, and not caring how you get there, but what you DO one you arrive.
I'm not a huge hard SF fan myself, though I can understand the appeal.
Now, some may think that I'm leaving out whole genres of SF games. Cyberpunk, for one. But I think that's a modified space opera, or possibly a modified hard SF game, depending on in what direction you take the game/system.
I have no clue what it might take to produce a good SF RPG, since it's not my thing. I'm a fantasy fan; I rarely play SF stuff. But when I do, it's more science fantasy than science fiction, if you understand the difference.