Because lisp is an immensely flexible language/concept. Everyone has their own idea of what it should be/can be.

Common Lisp was exactly what it says "Common". Just some sort of common ground between all the various lisp dialects that existed, because everyone went their own way in extending the language. People end up coming back to it time and time again, because all the others don't quite provide that common ground for people to work on. Whilst there may be things missing, there's always something more missing in other lisps.