Almost no languages are built for the environment where all code and values must be live and persistent and accessible to many people at once. Sandboxing and permissions need to be much more granular than what you get in most implementations, and persisting complex state (nothing in-world can disappear or break just because a server is restarted) is hard if that state is not first-class in a language (imagine a half-consumed generator in JS). There's more, but in short: the requirements this environment enforces on the "mudlib" language are unique enough that no mainstream language is a good fit out of the box.