You're underestimating how familiar someone's email address is versus an OpenID URL whose significance the user doesn't know and whose use she can't grasp.
Agreed. URLs as an identifier are completely alien to non-technical folks. Even I think the notion is odd. They just don't make any sense. Plus they are hard to type correctly. Email addresses don't have these problems.
One of the reasons why we couldn't just "fix" OpenID is that we wanted a scheme that would be privacy-sensitive.
With OpenID, the result of the site redirecting you to the IdP (and then the IdP redirecting you back to the site) is that the IdP can get a trail of every website you're trying to log into. That's pretty fundamental to the way OpenID is designed.
