I'm actually still running a MUD (Genesis, the first LPmud) and one of our main issues is that we don't get many new developers who want to help out and code. Nowdays players just want to play, getting them to go through the extra effort to learn to code and help out developing the world is very hard.

It's also hard to get new players but a web client has really helped with that. Very few new players stick around though. But enough to keep it going. The lack of developers is probably what will kill the game eventually.