Because increasing its importance could be seen as tampering with the results for their own benefit, i.e. pushing a web-technology that they control. Adding a flag to it is sort of the same, but I'm guessing they are testing the waters with this and if they don't get legal repercussion, they'll actually change the search rankings.
No I'm not saying they increase the rankings of fast AMP pages. I'm saying increase the rankings of pages that are fast regardless of the technology they use. You can have a fast page with normal HTML.
