What keeps back Mozilla to implement this setting and lobby for a general Web API for expressing cookie consent? As far as I can tell, their users would be extremely happy about that.

Because it won't take off. Right now, the advertisers are basically hoping for you to be too lazy to click around ten minutes to find the 'no'-option. If every user would be presented with a fairly weighted chance once, hardly anyone would click yes. Accepting this standard would undermine their business even more.

That existed. https://en.wikipedia.org/wiki/Do_Not_Track

It failed horribly because it was voluntary. But now that it's a GDPR requirement, perhaps that might have a snowball's chance in hell of succeeding.

No, it failed because it acts on the wrong side. If you don't want to be tracked you shouldn't send cookies in your request.

It can, first iteration used the Do-not-track header, but that died in the standardization process, now there are a movement for the Global Privacy Control header that you can read about here: https://globalprivacycontrol.org/