I think the only real answer to that is if you think OpenID is likely to confuse users during signup and therefore you loose users. I think even though its early the increase of signups is likely to be greater than the loss of users through confusion. Especially if implemented well using Clickpass ;-)

Also another answer is that it is currently not completely trivial to implement, so if you don't think the benefit outweigh the time to implement than you wouldn't. Hopefully we will help fix that soon as well.