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.