In case anyone's curious, what happened was that Dave (@davidmarin) and I (@irskep), the mrjob maintainers, left Yelp within about a month of each other. (There's no story there, just coincidence.) There was never any momentum with new maintainers, going by the release history.

But now Dave is working on mrjob regularly again, hence the pace of recent improvements.

Grandparent is correct about the second-class support for non-EMR production Hadoop usage. Like any open source project, the code only works well if a major stakeholder invests in improving it. Few non-EMR users spend much time contributing, so the situation doesn't improve.