I would if I could, but there's just one problem.
Jeff is right.
Sure, it's true that fantasy has a head-start. And when I say fantasy, I mean high fantasy. Elves, dwarves, women in chainmail, all that lovely shit. The game mechanics that have proven to be the most successful (i.e. fun) have been a part of fantasy-themed games. Meanwhile, sci-fi games have struggled with making combat involving and interesting, and are still getting a grip on the whole "MMORPG players want epic fights/bosses and dungeons" thing. Basically fantasy games have been more fun, less hassle.
The fact that the first successful MUDs (or the first MUDs for that matter) were fantasy-based also doesn't hurt. People are going to take what is successful, including its genre, and build upon it. And let's not forget the short but generally true observation that girls don't like robots. Apparently they don't need anymore big pieces of junk with artifical intelligence that can't keep control of their nuts. They get enough of them in the real world.
Yet, and follow me on this, if we were to take two games - one fantasy based and one sci-fi based - which were virtually indentical in terms of comparable gameplay and game mechanics, the fantasy game would still obtain a higher subscription number. Take 10 games, 5 from each camp, and you'll have your top 5 MMO slots being taken up by fantasy MMOs.
It all comes down to our past.
Growing up, fantasy themed stimuli is all around us. We walk amongst it. We breathe it. It is a huge part of our learning, of our fun. From the fairy tale books we are shown to the tribal and social legends we are taught, from the school plays we perform to the customs and history of our race we learn, we are constantly bombarded with fantasy themed and fantasy-esque stimuli. As a result, fantasy becomes easy to identify with. It's a safe bet for the player, and it helps recreate the fun we experienced all those years ago when we were imagining ourselves as the knight in shining armor or the most beautiful princess in the land.
This is not to say sci-fi isn't interesting or fun. Hell, it's a hoot. Just take a look at all the people blasting off to distant planets and ganking exotic aliens in well-established licenses such as Star Wars: Galaxies, or less well known yet still successful titles as EVE Online. I <3 my sci-fi.
But the fact remains that people find it easier to see a hero or epic journey in a fantasy game and place themselves in that role. People want to be the hero. The fact that most fantasy mediums (books, movies etc) seem to revolve around a hero saving the day against unbeatable odds whereas sci-fi mediums tend to question what it is that makes us human or say "Gee. We're really only a small speck in the eye of the universe" only tips the scale towards fantasy MMOs.
So while I feel and hope that there is still much room for successful and acclaimed sci-fi MMOs, it's going to take alot of work on the part of the sci-fi MMO makers to distinguish their work from the norm and raise their baby up to the level of their Wizard cousins. It may even have to involve MMO makers taking apart the sci-fi genre, discovering what really makes those fantastic sci-fi stories so interesting, and including that pure gold in their MMO. Merely swapping swords for guns and horses for spaceships won't cut it.
Meanwhile, as long as it takes to get to that point, the top spots of the MMO ranking list are firmly in the hands of the fantasy family.