There is something about Democrats, and many liberals in general, that makes them desperate to be seen as reasonable to the exclusion of everything else, even winning.
Republican obstructionism, hypocrisy, and plain old hate have taught Democrats nothing these past 3 years. There are some signs of life in the Obama campaign, but no evidence of a permanent change learned by the fire of his enemies' hatred.
If I thought there was some great benefit to this, I might agree, but there is ample evidence that nobody ever sees them as "reasonable" and that they get nothing for their attempts.
Barack Obama didn't win big because of his "reasonable" platform. He was widely seen as either a liberal change agent or someone who could make the other side agree with him by sheer force of his personality -- or both. Now, that never made a lot of sense, but it hugely appealed to a large number of people. They didn't love him for being "reasonable", they loved him for being powerful. That was a rare race and he was a rare candidate.
As it turned out, he too wanted more than anything to be seen as the grown-up in the room, splitting the differences, making Grand Bargains, mediating between the two extremes and most of all, "changing the tone" which was a fool's errand, but it didn't stop him from trying.