There was a vicious spiral at work related to advertisers not caring enough about page load, and adding that signal was not enough. AMP shifted the game by placing the burden on Google of specifying a way to hit advertisers' goals while offering a faster experience.
https://moz.com/blog/google-amp-search-results https://searchenginewatch.com/2016/07/06/is-google-amp-a-ran...
[just to be clear, just because you read it in some HN thread doesn't make it true. Let's see a single citation that it actually influences ranking]