Other than StackExchange, I can think of no other major site who looked at the over-engineered protocol, the implementation headaches, the confusing user experience (nascar board of provider logos), and said "yes, that is definitely what we want to have rather than a validated user email address."
How would that work for sites like Hacker news, reddit, and twitter? Does every single user have to preview and approve every single other user on the website? That doesn't scale at all.
These days with browser/mobile sync, maybe it's actually possible. But like a synced password manager, it makes a primary account breach that much more devastating.
1. Education for and purchase of U2F keys 2. Key loss recovery mechanism 3. Key stolen defense (you can't just rely on the U2F key alone, there must be a pw or other type of second factor) 4. Widespread browser & device support (without it, a user/pass is required as a backstop)
Nevertheless, it is progress.