"Leaving it up to the community" - we all know what that means. Even if it doesn't mean it though, what image does that leave? Sure, they use it as their internal auth system, but nobody outside mozilla cares about their internal auth.
Mozilla is in favour of the open web. An open web needs properly open third party authentication. They obviously know that since Persona does things right. It seems like, since it didn't gain the traction they wanted it (in my opinion due to bad marketing and bad prioritizing than anything else), they're just giving up.