It's weird how fantasy has such a lead over sci-fi in RPGs.
D&D being the only RPG with broad name recognition probably has something to do with it.
My feeling has always been that sci-fi is just more varied than fantasy. Sci-fi/fantasy is often presented like a single genre, and one defining trait is that it asks you to accept one preposterous premise to make the story work. Fantasy asks you to accept magic & monsters, and is generally in an otherwise medieval setting. While there's a wide range of fantasy settings and stories, they have quite a lot in common, especially if you're willing to ignore any details about how magic works - a dude with a sword is not going to be out of place in many of 'em. Sci-fi stories, OTOH, often pivot around a different, more specific preposterous 'What if...' notion. There are bits and throwaway details that a lot of sci-fi recyles, FTL travel, bug-eyed aliens, ray guns and so forth, but it's not just that the details are different, but that the differences seem harder to ignore. And, even if you have all the throwaway details in approximate alignment, they can still feel very different. Star Trek and Star Wars both have humanoid aliens, ray guns, and FTL, but they could hardly be aped by the same RPG. Tolkien and REH, OTOH, both get used as inspiration for D&D with few issues. Dude with a sword works. Dude with a raygun is different if his ray-gun is a phaser than if it's a blaster.
So you really can't have a more or less generic SFRPG the way you can an FRPG like D&D, they're all condemned to more niche interest. If they try to cover a wider range, you end up with something like GURPS Space, Space Opera (which neatly illustrates how an amalgam of sci-fi from the 50s-70s just doesn't work like an amalgam of fantasy from the same period did for D&D), or Star Hero.
Edit: Actually, there's one sub-genre or tangent of sci-fi that does work for an RPG: superheroes. Each super-being gets to have his own preposterous 'what if' conceit, so you have an amalgam of all sorts of sci-fi ideas and bits (and fantasy for that matter) and it all sorta works within the weirdness of the genre. Champions! and Mutants & Masterminds are only two examples of good, reasonably successful superhero RPGs that are fairly generic.