I think it's more than that - after all, most of the better quality muds have fantasy themes. Fantasy muds seem to have a greater following, but I suspect a big part of the reason is combat.
Combat plays a vital part of most muds, but most muds don't support ranged combat very well (if at all). A fantasy theme would typically use melee as the standard mode of combat, but for a sci-fi theme the main focus is almost always going to be on guns. And you really don't want the primary aspect of your game to be poorly supported.
That's my theory, anyway.
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