Hilary, because it was already a Democratic year, and Romney taking a pro-choice female as his running mate would absolutely positively split the Republican Party. Seriously.
Huckabee delegates will walk out of the convention in St Paul and support a third party candidate. This was threatened if McCain took Lieberman as his running mate, and Lieberman is well known and popular with the GOP conservatives.
A Democratic ticket with two women on it is nothing compared to that.