Because they chose the wrong API (well more correctly created a new, worse one).

In order to appease the various flavours they mixed and matched stuff from Tornado, gevent, etc.

They should have stuck with the most seamless of those (gevent) and instead of having it monkey-patch the runtime go the Java VirtualThread route and natively yield in all the I/O APIs.

This would have given a Go-esque ease of use and likely would have been immensely more popular.