There is a fundamental misunderstanding about popularity. People think that popularity is directly related to merit or rationality.

Technical things are largely popular for the same reason non-technical things are popular: trends. In other words, they are popular because other people perceive them to be popular. Humans are herd animals.

async is harder and associated with Node.js/JavaScript which probably makes it uncool for a certain influential python subculture.

But actually Fast API has basically taken over and now I think people should recognize that means async IS popular in python at this point.