Why Fantasy MMOs Have An Edge Over Sci-Fi

Does the Sci-Fi genre mix well with the MMORPG? The first MMOs were fantasy, based on the old Dungeon and Dragons role-playing games. The rules and terminology in MMOs, even recent ones like AoC and Warhammer, still hark back to the pen and paper era. Sci-Fi MMOs have to take that basic formula and change it to fit their universes. It’s relatively easy to do, replace mana with energy cells etc. but is it fun to play once the mixture has been tampered with? I had a look through the brief history of Sci-Fi MMOs to see how they fared.