@Judas Escargot, Acerbic Prophet of the Mighty Potato God: I think part of it is simply that Obama has the superior ground game, so even if Romney is close, right now Obama is doing a better job of getting his supporters to the polls early.
Romney may be relying on the idea that his base will get itself to the polls on election day. Part of the problem for him, though, is that every Obama voter that votes early is 1 less person the campaign has to worry about going forward. Which means they can expand their universe of targets between now and election day.
