Because everyone has Google, and the lowest setting on Sign in with Google+ contains some personal information from your account, including things like DoB, Ful

Because everyone has Google, and the lowest setting on Sign in with Google+ contains some personal information from your account, including things like DoB, Full name, etc. Google recommends OAuth2 over OpenID for authentication, and I'd rather be using the service Google supports the most in order to provide the best experience to users. Just because you use OpenID and Persona doesn't mean that the majority of people will. But the majority of people will have a Gmail account, or a Facebook account.