[jfasi]

I think the reason is both technical and social.

The JVM has a very hard time integrating existing C code and extensions. For instance, in the case of python, Jython lacks the ability to take advantage of the significant number of modules written in C.

Consider what this means for performance. Imagine a pure python module that attempts to import a C submodule for performance, and falls back on on the pure python implementation specifically to allow it to run on non-CPython implementations. json is an example of such a module.

This module was likely engineered and architected with C module optimization in mind. I'd assert that in most cases the pure python module is a crutch to allow a CPython-targeting module to run on other implementations.

This leads into the social reason. Community takes a long time to build and using the JVM for dynamic languages is a relatively new trend. The JVM is a very mature, well-optimied technology for static languages. It is only now that dynamic languages are starting to gain traction on it.