I like swords! (Welcome to Corneria!)
Combat in FMMO's tends to generate around a few similar aspects, your melee combatants hacking away at a target with a myriad of sharp pointy things, or bashing them with blunt heavy things. This style of combat is (generally) easy to produce, as it doesn't factor in things like spell range, line of sight, and all of those other pesky ranged problems. But you then also have your caster types, which for all intents and purposes are the gunslingers of the fantasy genre, being able to fire off spells from range, and duck behind corners to avoid things thrown back their way.
So it certainly couldn't be the combat style, because when you look at the combat in Tabula Rasa lets say, aside from a few obvious points (bullets are not fireballs) it's essentially the same thing as a mage tossing out spells. And not to mention TR has its melee classes as well in the sword wielding spy and staff wielding guardian.
Could it be the actual style of fighting that makes the difference then? Could people just prefer battling dragons and ancient gods, instead of wave after wave of Stormtroopers, or Bane? Does the prospect of losing your ship and never getting it back turn away people who would much rather just have to run back to their beaten and broken corpse?
Or maybe it's the healing factor? It makes more sense in a fantasy MMO for a priest, shaman, cleric, white mage, or whatever to be standing in the back row tossing out endless amounts of warm-fuzzy mojo to keep the party alive. Than to have, say a medic running around, hopping everyone up on morphine and band-aids to stop them from pouring their intestines out on the floor.
Certainly there has to be some weight in the combat differences. I for one felt odd playing the TR beta, shooting at bane after a half a decade of first being a sword and board wielding paladin in FFXI, and then a shadow-magic tossing warlock in WoW. It's certainly not that I'm more drawn to fantasy than sci-fi as outside of my MMO's I'm a huge sci-fi nerd.
Story time! (Get your PJ's on)
Lore is a strong contender for why FMMO's might be more popular than SFMMO's, with Tolkien's amazingly rich tapestry behind Lord of the Rings Online, the brilliantly built lore that backs Blizzard's World of Warcraft, and Robert E. Howard's characters and stories filling in the upcoming Age of Conan:Hyborian Adventures, there's no doubting that each is built on a very solid foundation of story and myth.
But then you look at the lore behind Star Wars Galaxies, which is hardly anything to scoff at. There are easily more books, role playing games, and even movies dedicated to Star Wars than there ever has been to WoW. And it would be unfair to say that the stories that come out of LotRO are more epic than the story of Luke and Co. running off to save the galaxy from whatever nasty is threatening it this week.
Maybe it's the fact that Fantasy lore just has a greater romanticism to it, we are (mostly) all brought up with tales of princes and kings, knights and dragons, and swords and sorcery. Whereas most of us don't truly discover Sci-Fi until we're into our teens or beyond, generally because the genre is a little more involved (it puts the Science! in Science Fiction). So perhaps it's just a deeper seeded appreciation for the story of the valiant knight vanquishing his foes, instead of the pale cheeto loving hacker who shut down the opposing corporation's ship yards?
Whatever it is, there is one thing I'm glad of though, and that's that developers aren't giving up on other genres than fantasy. With Jumpgate Evolution on the horizon, Stargate Worlds already enticing people, and off-the-wall titles like Lila Dreams under development, it makes me think that eventually there could be 10 million subscribers bouncing around between planets, dressed in space age silver fabrics, and saying things like "Frak!", "Live Long and Prosper" and "Just like shootin' womp rats, eh kid", instead of "I AM THE LAW GIVAH!", or "LEEEEROOY..." well you get the idea.
Let's just hope the developers keep pushing out new and ground breaking content, as long as they give me a sword to swing around every few years or so.