Also, if the app hasn't been updated for years, chances are that it targets and outdated version of Android, which works, but is considered suspicions by Google as it is also a way to bypass some security checks in recent Android versions.
There are probably many other criteria but I think these are the most likely.
Not saying it is a good thing, but it explains the reasoning. I wish Google did human reviews and not rely that much on their bots, but then that's what Apple does, and it is even more locked down. Maybe try F-Droid, it is an alternative app store dedicated to open source.