Twisted ROCKED. I remember when it came out everyone I knew thought it was pretty kick ass, at least those who developed in Python.

Did something happen that it's now fallen out of favor?

By now, people expect libraries to have thorough documentation on the Web, not just an O'Reilly book.

The fact that the port of Twisted to Python 3 is slow-going, and far from complete, also gives the suggestion that there are corners of the code that developers don't even understand anymore.

Twisted is still popular. Problem is that in 2.x the async ecosystems do not work together and many things might not exist everywhere. Twisted seems to have lost a bit of ground with newer things like Kafka etc.