I think that Fantasy, as a genre, has had several inherent advantages over other genres that have generally lent themselves to being the predominant format of RPG's. Here's my argument:
- It's accessible. Aside from the whole LotR thing, the notion of knights and wizards is pretty embedded into Western culture. You say dragon, people get what a dragon is. You say Andorian, and you'll get a blank stare.
- No specific licensing. I'm of the belief that licensed RPG's are a mixed blessing. Sure, you appeal to the fan base and have a ready, pre-made introduction to the world, but you're usually hindered by both trying to reproduce the "feel" of a movie or a book in a gaming context, and that is not easy to do. Not to mention that you'll have to figure out how the PC's are relevant in the over-arching plot and not spear-carriers for the licensed story (I'm looking at you, Star Wars). Yes, there are licensed LotR games out there, and historically they haven't done well. I just think that a generic fantasy setting is more accessible than a generic sci-fi one, mostly for the reasons in the first argument.
- Technology is a pain in the butt. This is complex issue, but let me see if I can break it down. The old slogan about the gun being the great equalizer is generally true, and the issue facing a lot of modern- to futuristic-games is that you someone have to maintain a semblance of that realism--the notion that anyone could be shot once and killed--with the scaling effect generally in place in RPG's. By that I mean that you want big, bad guns who require more effort and present a greater danger as part of your story, but a big gun can generally negate that greater threat. In fantasy games, where opponents tend to be often non-human and weapons tend to be more of the "it'll take you eighteen cuts before you kill the guy" it is easier to structure games where the BBEG at the end really will take a while to take down.
- Then there's the transportation issue. On fantasy, you're largely on foot--you're or some animal's. Having a rough idea of how far the PC's can travel give the GM is a little bit of not just control but necessary foresight. Barring some pretty high-powered spells (that you would know your PC's have) you can say to yourself, "okay, they could go to that ruin, that temple, or that den. That's what I need to stat out." Contrast that with say, an RPG based on my life, WQRobb, regular human being. You think I'm just going to kick around my home town all day writing on my blog. Forget that, I'm scraping together all my savings, buying a plane ticket, and flying to Aruba. Highly unlikely? Absolutely. But possible. And players are always doing things you don't expect. Restricting travel by dint of genre helps rein that in, even in a small way.
- Classes. Call them "Occupations," call them "Archtypes," call them whatever you will, but most fantasy RPG's have fairly delineated character concepts, each of which perform a fairly vital role on the team. Everybody's got a job, and there's not a whole lot of overlap between them, even if you're running 4+ players. Modern and futuristic games tend to be skill based, less delineated, and prone to overlap. It is my general impression that most sci-fi games who try to avoid this problem end up doing so by introducing elements that are often associated with fantasy--psychic powers, weird alien stuff, etc. And highly-delineated character concepts aren't necessary given the right group of players, but they do help in making every player feel valuable, which means they are having a better time at the game.