The reason for the URL as username is that OpenID originated on Livejournal, where users have their own URL (i.e. mine is andrewducker.livejournal.com)

It therefore made sense to use URL endpoints as identifiers, as you could bounce people to their authorising server incredibly easily. Doing it via email address would be much harder (where would my email, andrew@ducker.org.uk, be authorised by?).

It's caught on amongst people who have URLs (bloggers, journallers, etc.. It hasn't caught on amongst people who don't (everyone else).