Because much of the power of python comes from its libraries, many of which depend on C(++) extensions tied to details of CPython interpreter. It's very hard t

Because much of the power of python comes from its libraries, many of which depend on C(++) extensions tied to details of CPython interpreter. It's very hard to write a JIT without breaking this. For example, all the numerical/scientific stack (numpy etc) is extremely fast by being written C, Fortran, etc with a Python interface. If you break this in a JIT then the plain python might be faster but actual useful code will be much slower. Additionally, one of the reasons C-extensions are so fast is because the CPython implementation is very clean which has made it easier to write stable and performant extensions.