Comment by samcolvin

Victor Stinner (Python core developer focused on performance) has a great speech about this question:

There are a lot of reasons actually

- Performance limited by old CPython design. If you fork it you have to deal with all the legacy code.

- CPython is limited to 1 thread because of the GIL.

- Specific memory allocators, C structures, reference counting, specific garbage collector etc.

You can find that video in here: https://youtu.be/TXRPCZ7Nmh4