It was supposed to bring massive concurrency to Python. But as with any async implantation in any language it is too easy to deadlock the entire system. Did you forgot to sprinkle enough `await`? Your code is blocked somewhere, good luck hunting for it.

In contrast preemptive green threads are too easy. Be it IO or CPU load all threads will get their slice of CPU time. Nothing is blocked so you can debug your logic errors instead of deadlocks everywhere.

Async works in JS so well because the entire language is designed for it, instead of async being just bolted on. You can't even run plain `sleep` to block, you need setTimeout.

It is even funnier because JS only got proper async after, what? 25 years or so of existence. The main reason JS went all in with async is because it only ever had a single event loop and that naturally fits with the async model.

I still remember the days when all the libs started adopting async and how so many of them (to this day) support both passing callbacks or returning promises. Async just so naturally fixed the callback hell of 2010s JS that it just became standard even though it is not even heavily used in the browser APIs.

What I find funny is Java started with green threads, then moved away to system threads, then back again.