I’m not an expert on this but it seems like they just didn’t want to find a way to give you green threads without GIL. This had already been done in another Python implementation: https://en.m.wikipedia.org/wiki/Stackless_Python

Stackless has a proven model. Basically the same model as goroutines and channels. It’s the reason EVE Online is able to run its primary game server in Python with such a large number of users.