Because not everyone wants to be a part of the asyncio trend.
Asyncio in Python is a poor feature that splits the language's ecosystem into 2 mutually-incompatible worlds, something Python only gets away with because it's too big to fail.
Meanwhile we've had Gevent for decades now. It gives us async that you can forget you have. Because rather than making code async, it makes the VM async.
Gevent could have been merged into CPython, but they chose explicit "structured concurrency" and the rest is history. History of sometimes moving forward and sometimes straying from the path and getting lost.
And lost Python's asyncio is. PDB, which lots of other debuggers base on, is still broken (cannot use await). The ecosystem? IPython uses asyncio internally so it cannot easily be embedded in a working async program. The only embeddable REPL I was able to find is this: https://github.com/prompt-toolkit/ptpython/blob/master/examp...... actually, it looks like someone is working on adding `await` support to PDB now, years after asyncio's first release.
Overall, lots of churn to get something (maybe) as good as Gevent, which we had in Python 2.7, or even before.
If a similar amount of effort was spent on first-class support for code hot-reloading and live program inspection, we would get a massive boost of productivity. But somehow even otherwise bright people choose to reimplement working solutions into something objectively worse, meanwhile our development/debugging loop still emulates loading punchcards into mainframes.