Jay Cost at RealClearPolitics explores two reasons for Obama's polarization:
The hook of [Obama's] candidacy was: America, do you really want to do Bush-Clinton-Bush-Clinton?
Yet here we are, breaking records for polarization. How did that happen? Why has Obama failed to do what he promised?
I think there are two big reasons.
First, Obama's implicit claim throughout his candidacy was that public divisiveness was somehow a failure of leadership. This was mostly nonsense. This country has been divided over cultural issues since at least 1973 and Roe v. Wade. It has been divided on fiscal issues since Reagan cut taxes in 1981; this ended the hidden tax of bracket creep, but meant that legislators had to make hard choices between more spending and lower taxes. It has been divided on foreign policy issues since the Bush Administration's response to 9/11.
These are all real things. They are not rhetorical wrinkles that a Jon Favreau speech can iron out. Obama's choices have mostly been liberal (with the notable exceptions of dealing with Iraq and Afghanistan). His speechwriters have endeavored to present his choices as win-wins, but their words have failed to persuade because the President's choices are rarely in fact win-wins. They usually favor one worldview or set of interests over others. Favor one side enough times and the losers will start to see what's going on, "eloquent" speeches aside.
Second, insofar as leadership could bridge the many divides in this country, this President has never been in a good position to exercise it. He owes too much to others. You don't win a nomination battle like the Clinton-Obama smackdown without making a bunch of promises. Remember that neither Clinton nor Obama secured enough delegates through the primaries and caucuses; Obama needed the superdelegates, chief among them being Speaker Nancy Pelosi (easily the most powerful Democrat in the country prior to the President's inauguration). There is a long line of constituent groups in the Democratic Party who certainly needed assurances about what an Obama presidency would look like. So long as reelection remains to be secured, these groups at least have to be monitored if not placated. And so, in a time of great divisiveness, the people with the closest connection to the 44th President are consistently on one side of the aisle. The left side.