That reason is Iraq

The American Scene:

The debacle of Iraq has completely changed the way many at least some on the left talk about foreign policy, has midwifed the birth of a kind of left-wing realism. It is the left (or a portion thereof) that has taken up the mantle of the Reality-Based Community. In the same way that some conservatives (Larry Kudlow, for example) have begun to embrace a kind of unified-theory-of-nonsense embracing neo-creationism, the idea of tax-cuts as revenue-raisers, and the conviction that we are winning in Iraq, I believe there is an emerging group of left-wingers who are going to take empiricism seriously across the board, and not just when it comes to scoring points against the Bush Adminstration.