I've been wondering about this recently and haven't seen a good answer. The question might be better put, "Why doesn't Vim support other languages for scripting?" I've found Vimscript illegible and difficult to debug, and the thought of implementing a meaningful project in it scares me off. Emacs has lisp, is there a reason Vim is stuck with Vimscript?
It is probably primarily due to age and inertia. It is at version 8 now, an eternity in software years. While it might be easy to bind your favorite scripting language to an app today, that surely wasn't so straightforward when "vim" began. And once more than a few people have invested great effort to create useful plug-ins, you would not be helping anyone by trying to completely rewrite the extension mechanism.