You can even compare it to OpenID, where it languished mostly-unused—but still actively-maintained!—for 5-10 years before OpenID Connect (a hybrid of the ideas of OpenID and OAuth) became the basis for all the major Single Sign-On services in use today. I doubt OpenID Connect would have inherited any OpenID-like functionality (e.g. profile discovery) if OpenID had been deprecated a year after its drafting. They probably would have instead (badly) reinvented it with no knowledge of the prior art, because there wasn’t an existing (if small) install-base to prove out the practicality of the various features.
Why is that a problem, by the way? Wouldn't we expect something like Persona to be implemented in new software projects first, then gradually get adopted in existing software?
Nobody is going to gut their auth system to implement something shiny unless many are asking for it - i.e. by saying "oh it's so easy to sign into $rival_website because they use Persona, why can't we get that here?"