While I have mixed feelings about Cloudflare, I don't see why you have been downvoted. This is on topic for the discussion, already implemented for a couple distros, etc.
The question of "who is responsible": anyone is free to run their own mirror, after all this software is freely redistributable.
As for "why not", if I were to lead a project like Alpine, I would insist that the org stays in control of its own infrastructure. Mirrors are also only one chunk of the problem; you also need builder machines.
presumably because it's a silly idea, given Cloudflare isn't a colo or dedicated server company and they won't let you just rack machines in their DCs for the same reason Google won't.
But i don't know who is responsible for that.