One thing I meant to mention in the sci-fi post but didn't: not only are the sci-fi subgenres more distinct and their audiences more fragmented than pulp fantasy, but not every sci-fi subgenre has a generic setting available.
Someone said in one of the comments on the Grognardia post that post-apoc sci-fi is the most popular form of sci-fi gaming. That may be the case, and it may also be true, as the commenter went on to suggest, that it's the most popular because it's the most D&D-like. But post-apoc sci-fi is also notable in that it has a generic setting. A lot of post-apoc movies and TV shows look alike, mainly because they copy the Mad Max movies to some extent: desert wasteland, weaponized jury-rigged vehicles, punk or metal fashion. The same is more or less true for cyberpunk (large criminal class, sprawling urban settings, resource shortages, evil corps, computer links everywhere) and early stage space games set on Earth (rockets, small colonies, frontier-like feel, asteroid miners, space pirates.)
You could run a game in one of those settings without too much advance prep, winging new details as needed, much the same as you could do with the generic pulp fantasy setting: just run with it, deciding politics, culture, and even some technology as you go along. You don't even need to worry if players have seen the same movies or read the same books. It's harder to do that with space opera or sprawling interstellar empires, because you have to establish what's available tech-wise, and because so much of the action is based on political maneuvers, so players need to know something about interstellar society.
You could play in an established setting, like Star Trek, Star Wars, Dune, or Battlestar Galactica, possibly filing off the serial numbers and just using the source as a reference point, so that players know what they can do and what to expect. But that depends on all the players knowing the source material and liking it enough that they'd want to play in that setting. I'm pretty well-versed in original Trek and some Battlestar Galactica, but don't really care for Star Wars, know very little about Dune, and less about practically every other sf franchise. Other people are going to be about the same, but with the settings arranged in a different order. This is why it's very hard to form a large base of players for any one particular setting