IncGamers brings us a quick article that examines some of the advantages that fantasy-based MMORPGs have over their sci-fi brethren. I can't say I agree with all of their conclusions, but I suppose some of them make sense:
MMOs give players a change to be someone else for a while. When making a fantasy character, the role you choose has an effect on your abilities. The gear you pick up increases those effects, but it's your character that swings the sword, pulls the bowstring or casts the spell. In Sci-Fi, many characters rely on machines, either guns or ships, to do their work for them, and I personally feel that takes something away from characters, makes them less heroic. And what you're doing in-game often isn't that far from the truth - I'm sitting at a computer pressing buttons, playing a character that's sitting at a computer pressing buttons.
Then you have the environment. Fantasy MMOs normally tend to be set in olde-world locations. Green forests, marshlands, farmland, mountains and caves, places many people see as idyllic. When you finish your adventures, there's a home city waiting for you with warm taverns and cold ale. What could be nicer? Certainly not the clinical space stations or futuristic minimalist areas that supposedly await us in the furture, according to Sci-Fi. And that's if things turn out well in the years ahead. There's the post-apocalyptic option, as seen in Fallen Earth, which has players scavenging for food and goods worth using, or the high-tech war-time future as seen in Global Agenda.