In order to break up the monotony of continual clone project posts, I thought I'd muse a little on the subject of science fiction RPGs, riffing off James Maliszewski's post from a couple days ago.
I haven't played many sci-fi RPGs. I played Traveller a little -- not back in the day, but I played in a session using the original rules a few years ago, followed by a session or two of the GURPS version. I played Fates Worse Than Death (a sort of cyberpunk setting) once or twice and ran a GURPS Atomic Horror session. Most of my gaming has been fantasy or supernatural horror; I just prefer those to most forms of sci-fi.
And I think this hits upon a central reason why sci-fi RPGs haven't been as popular as fantasy RPGs: sci-fi isn't a single general setting, but contains many different sub-genres. The original fantasy RPG, on the other hand, presents a general setting, which can be altered if a particular sub-genre is desired. As James mentions, pulp fantasy is naturally very expansive, acquiring elements from practically anywhere, including sci-fi. Sci-fi, on the other hand, is kind of meaningless as a label; you have to pick a subgenre, like space opera, imperial space, cyberpunk, post-apoc, rocket patrol, or atomic horror.
And not everyone likes every type of sci-fi. I tend to not like the space opera and imperial space forms of sci-fi. I do like a narrower rocket patrol-ish approach, or atomic horror... which is why I want to create RPGs for those two subgenres at some point. But I don't expect them to be popular, because they are narrower genres than pulp fantasy.
Think of it this way: instead of comparing Traveller to D&D and wondering why it wasn't as popular, you should be comparing it to Spelljammer or Dark Sun or Planescape: specific settings under the general banner of D&D, in much the same way that Traveller is a specific setting (or setting template) under the general banner of sci-fi. GURPS Cyberpunk is another specific sci-fi setting, as is Gamma World. You can't ask why one of these didn't sell as well as D&D any more than you can ask why Frank Zappa didn't sell as many records as rock'n'roll.