The most successful MMORPGs all have a medieval fantasy setting, some sort of sword and sorcery. Ultima Online, Everquest, Dark Age of Camelot, Lineage, Final Fantasy XI, World of Warcraft, Lord of the Rings Online, Vanguard, Age of Conan, and the upcoming Warhammer Online all fall squarely in this genre. The market share of non-fantasy MMORPGs, like Star Wars Galaxies, EVE Online, or City of Heroes / Villains, is tiny. Historical games like A Tale in the Desert have never really taken off, and if you look closer at a game like Gods & Heroes : Rome Rising, it is closer to a Roman era fantasy game than real historic. Pirate games seem to be better placed for success, there are several of them appearing in 2007. But where are the MMORPG where you play a gangster in Chicago in the 30s, a World War 2 soldier, an archeologist in a Cthulhu horror setting, or a detective fighting crime in Victorian England? Why has fantasy such a huge market share in MMORPGs, while having a much smaller market share in books, films, TV or other media?
One reason is certainly the origin of online role-playing games having their base in pen and paper fantasy games like Dungeons & Dragons. But pen and paper roleplaying games quickly expanded in many other genres, and you could play anything from a Werewolf to the captain of the Enterprise. I've seen a lot less variety in online roleplaying games than in pen and paper ones.
The lack of success of other genre MMORPGs might be related to a lack of imagination of the game developers. Games like Anarchy Online or Star Wars Galaxies, while nominally in the SciFi genre, had gameplays that was very similar to fantasy MMORPGs, being limited to the surface of planets (SWG introduced space combat much later). In Anarchy Online even combat with laser guns worked pretty much like combat with swords in a fantasy game, leading to the crazy spectacle of a player avatar and some NPC mob standing toe to toe and shooting at each other with huge laser guns. Not very believable. EVE Online is doing better by having space combat instead. We'll have to see how well the MMOFPS idea of Tabula Rasa works.
The secret of making a good game in a genre different from fantasy is to not only change the settings, but match the gameplay with the setting. The upcoming Pirates of the Burning Sea will apparently resembe Sid Meier's Pirates more than World of Warcraft, and that is a good thing. Fantasy will certainly remain prominent in MMORPGs for years to come, but it is now probably easier to gain market share with different gameplay in a different genre than by making some sort of WoW clone. What genre would you most like to see turned into a MMORPG?