That's easy---because they did not like Romney or his northeastern brand of big government liberalism. The RNC's dumping all over the TEA party movement (especially in Tampa) and Conservatives (like Palin) did not help him either.
No it is not easy, that is the whole point. You do not know anymore than I do why white Republican voters stayed home. We do not even know, for example, whether it was white conservative as opposed to white Republican voters who stayed home much less why they stayed home.
I would bet David Axelrod knows.