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.

Of course, that's just one of the reasons.