It's because implementing this is not that easy: there are differences between the implementation of coroutines and await that makes it tricky (especially waiting for both CPU tasks and network events).

For Python I loved this talk by David Beazley:

https://www.youtube.com/watch?v=MCs5OvhV9S4&t=2510s

He's implementing async/await from coroutines ground up by live coding on the stage