Because it sucks compared to gevent (green threads). But for some reason, people always disregard this option. They don't even read it. Like any comment with gevent is shadowbanned and it doesn't register in their mind.

Gevent is too underrated. Even if people don't like the monkey patching you can simply use the gevent namespace API as well. No idea why people prefer the absolute mess that is Python async ecosystem.

Happily using gevent for our backend (IoT+ML) since 2019. Was very glad when I saw it is still being well supported by recent SQLAlchemy and pscycopg releases.

The fundamental problem with any kind of green threads is that they require runtime support which doesn't play well with any active stack frames that aren't aware that they are on a green thread (which can be switched).

I've used gevent for years and will probably never stop. I greatly prefer it (or Go) over asyncio.

People act like it's dead but it still works perfectly well and, at least for me, makes async networking so much simpler.