There is something about the language and/or community that leads folks to build their own X instead of collaborating on a common open-source version of X.
Perhaps the lisp learning curve is high enough to dissuade those who want to contribute to a project but don't yet know Clojure?
I have a pet theory that it's because it's too easy to write code in. I think there needs to be just enough pain with a language or library that it becomes worthwhile to allow someone else to write X and just deal with it being imperfect if it still solves your problem.
Perhaps the lisp learning curve is high enough to dissuade those who want to contribute to a project but don't yet know Clojure?