My priorities change.
Emails from my mother may usually be important, but something time-sensitive from a person who’s not emailed me before is likely to be even more so.
However much you try to automate these things, get GMail to ‘learn’ your preferences, ultimately it needs human input.
So I’ve turned off GMail Priority Inbox after playing about with it for a few weeks. Interesting concept, but not for me. 🙂