[Molly]

I'd say it's mainly due to player preferences.

For instance, my own Mud, with a Time Travel theme, has 4 different worlds set in different Timezones. And of the four, Medieval is by far the most popular, even though it honestly is neither the best nor most original of them.

We have a very large 'Prehistoric' world, which is mostly set in ancient Egypt and Greece around the time of the Trojan war. In my opinion that part of the Mud is superior to most of the zones in the Medieval dimension, (although of course there are some exceptions). It also has all the fantasy attributes you could wish for; myths and legends, swords and heroes, legendary monsters to defeat, like Scylla, the Hydra and the Minotaur, earth magic and witches in the old religion, the Olympian and Egyptian Gods who constantly meddle in mortal affairs - in fact the possibilities for interesting quests, plots, roleplay and warfare are unlimited during that era. There even are dragons, for instance if the Iason-Medea myth.

And yet the bulk of the players stick to mainstream, which means Medieval.
Why? My guess is because they are used to it. Players may claim that they prefer originality and unique settings, but the truth is that the majority don't. They generally prefer what they are already used to from other Muds, because then they don't need to think along new lines. So no wonder most Mud owners choose Medieval too, that's the safest bet.