It's much easier to implement async/await, especially in languages that don't have as pervasive a runtime as JVM and/or have substantial amounts of native code extensions that can't be managed, i.e Python/JS/Ruby.
https://github.com/python-greenlet/greenlet
has been available for quite some time in Python (gevent probably being its most used flavor).
Note that it it actually predates the async/await approach which was incorporated into the Python language (so, in Python it was implemented as a third-party library -- even async/await had an implementation based on Python 2 using yield and some decorators: https://pypi.org/project/trollius/).
I really do think Python should have instead adopted gevent as it's async approach instead of asyncio and async/await etc.