I think the reason sci-fi gaming hasn't caught on as much as fantasy is for one very important reason: in order to run a really good compelling sci-fi game, you have to know about real life science. I would love to play in a really good hard sci-fi game, but every time I have tried it, the GM simply lacks the knowledge to pull it off. Heck, I have a PhD in Neuroscience, know lots of neurological and biological info, have had a lot of psychological training, and am competent at chemistry, physics and mathematics- and I wouldn't trust myself to run a hard sci-fi game and not get boggled after a time (very likely sooner than later).
That brings me to another point I just thought of: hard sci-fi is hard to run since the GM has to be VERY familiar with the tech and possibilities in that universe. I've had some cool ideas for hard sci-fi universes, but the sheer detail needed to describe all the tech keeps holding me back and killing my enthusiasm for trying it. The GM in a sci-fi game HAS to have a deep understanding of the tech and what is possible, or else the game will lack any sort of believable or realistic quality, and the players lose interest fast (personal experience on this one, 3 times).
I'm a Star Trek geek, I have watched pretty much all the shows (except Voyager after the 2nd season- blah), and the Star Trek world is the only one I'd be comfortable running due to the development they have put into detailing the tech and what is possible. Unfortunately, Star Trek games don't usually play so well (having some people being superior officers to others is inviting trouble). Star Wars is VERY soft sci-fi, more resembling the Odyssey in space than sci-fi, and it usually feels incredibly railroaded and moralistic when I have played it before. Between the two you've got Fading Suns, which is a brilliant universe, and I'd love to run it. Only problem is that HDI brings new stuff out at a snail's pace, and it seems to be hard to get people willing to try playing it. Traveller and Transhuman Space both look like good hard sci-fi settings (I have the books), but the sheer amount of material the DM needs to be very familiar with seems prohibitive.