MUDs are coded such that anyone who knows how to write an area can do so. Implementors can choose to put in any area they wish, from an impressive selection. This can be done in minutes, or longer if the Implementor wishes for quality areas. MUDs are text-based, meaning that even a 286 with a modem will be sufficient for mudding. Games like The Realm or Ultima Online rely on huge Win95 16MB RAM computers to run.
MUDs are the equalizers in a world full of different speeds of computers and different incomes.
MUDs also take up no space on your hard drive. So MUDs are convenient wherever you are in the world. And finally, it's easier to learn to type fast and then pick up MUD commands which can be typed in at the prompt. Perhaps if these new online games offered a prompt to type in commands, we old-fashioned types would like them more. Banned ad server hosts.