Not affiliated with either project: PyPy is a completely separate Python implementation without perfect ecosystem compatibility. Pyston is a small fork of CPython that replaces/adds on a couple of areas. They're very different projects, and Pyston is the only one that could be meaningfully merged into CPython rather than replacing it outright.

For anyone curious, here's a set of benchmarks that compare Pyston vs PyPy (vs Python 3.11):

https://www.phoronix.com/review/python311-pyston-pypy/2

They're neck and neck in most benchmarks, interestingly enough.

As to why not PyPy ... well, PyPy is a complete reimagining of how Python is implemented, so a merge would be infeasible. It would have to be a very disruptive transition and the appetite in the Python world for a big transition is ... minimal.

PyPy is also slower when calling into C extensions, so that's one downside. I'm not familiar enough with it to speak thoroughly, though.

Because PyPy has never been a viable replacement for CPython engine in the confines of the CPython project.

For one, it would mean you'd have to throw away almost all CPython code.

Second, all the new code would be from totally different people than the CPython dev team (not very enticing).

Third, it would have compatibility issues with extensions and such.

Fourth, it would be slow calling into existing C code, which is like one of the major selling Pythons of Python (think pandas, numy, scipy, pytorch, and tons of others).

Unless something changed pretty recently, pypy's database support is pretty weak. It's not a drop-in replacement for Cpython.

This is a question to pypy folks.

The initiative in the message posted comes from Pyston folks themselves. They consider their code ready for that.