Because the intersection of the sets of people who (a) sign up for more than two websites a year; (b) know and care enough about security to avoid just using the same password on every site; (c) think it's much easier to use OpenID than to just use the "email forgotten password" link; and (d) aren't just using 1passwd or some other password-caching program is apparently really small.
One reason is that there's a very small distance between "I don't care if someone gets in here" (reddit, blog, etc) and "If someone got in here I'd be hosed" (bank, administration stuff, etc). OpenID can handle the former, but since people are using the same insecure password for all those sites anyway, typically, it doesn't matter. OpenID can't handle the latter, because (last I checked) making phishing easy is inherent in how OpenID works.
Because most that do catch on take a while, and some good things never do.
Mostly, people don't have a problem with using a username/insecure password. They want stuff and don't care much about security. Although I agree that if Google, Yahoo, MySpace, or one of the other humongous sites accepted it, that would spread the word much faster outside of the geek echo chamber.
No established site is going to want to use that for its authentication system. They don't want to make it easy for you to log into sites other than their own.
Of course, that's just one of the reasons.