I'd explain your cases this way:

C# has a dictator with a budget: Microsoft integrated async into C# in a formal way, with 5.0, including standard libs, debugging, docs, samples, clear guidance going forward, etc. What holes there were were dealt with in an orderly and timely manner.

JavaScript actually had a pretty messy start with async, with divergent conventions and techniques. Ultimately this got smoothed out with language additions, but it wasn't all that wonderful in the early days. Also, JavaScript started from a simpler place (single-threaded event loop) that never had "fork" and threads and all that comes with those, so there was less legacy to accommodate and fewer problems to overcome.

Python had a vast base of existing non-async software chock full of blocking code, plus an incomplete and haphazard concurrency evolution. There are several legacy concurrency solutions in Python, most still in use today. Python async is still competing and conflicting with it all. Not unlike the Python 2->3 transition.