It's just easier to outsource the authentication and not have to worry about passwords, encryption, resetting passwords, setting up 2 factor auth etc. Over the lifetime of MetaCPAN, maybe one person has publicly refused to create an account specifically because of this reason.

@larsbalker @metacpan That's all fair, and if I were implementing a new site today that had logins I'd likely do this too, with external authentication having become quite mature and standardized. But MetaCPAN was the first site I saw so far with that limitation so it seemed more unprecedented.