Because lets face it, both parties are corrupt and want a piece of that cash. As long as Citizen United exists the flow of money will be in both directions (but in one direction more than another, depending on the election).
Because money rules democrats will continue to shift right to get more of this cash but play lip service (as a progressive) to win votes.
Should it be repealed? Yes, of course. But like Campaign Finance Reform both sides have a lot to gain from Citizen United.
Obama is a center-right president and i can assure you he will not even bring up the idea of Citzen United being removed in 2013. The man is in his final term and its all about building personal contacts and growing his personal riches, as Bill Clinton did in his 2nd term. Republicans are jealous.