HTTP 2 can't just send content from other domains through the same connection (and any proposal that adds the ability, e.g. through signing, also can be used with HTTP 1.1 connections)

And pushed content needs to be accepted by the browser, just sending someone ad bytes doesn't really do anything useful. It doesn't impact adblocking, and if a browser vendor wants to take the adblocking features away they can also just do that for the traditional model.