They had already done that, perhaps for years. I don't know why it didn't work - perhaps there was a slowness oligopoly, where everyone was slow and no one was strongly incentivized to speed up.
If your marketing department want one more tracking script and your execs want one more pop up banner etc and each change is bad but not egregiously so, then your average "web guy" at most non-big-tech companies will just do what they're asked to.
With AMP they get to say "it's not possible"
People might have had more room to negotiate on how to hit a performance target but the thing which would have mattered was a number in the webmasters tools console.