I think its as simple as the fact that "we" won the Cold War, and that was the only thing putting any leftward pressure at all on US politics. The psycho right wing didn't materialize in 2008 out of nothing (think about LaRouchism, the "moral majority", Pat Buchanan, and on and on). Yes, Republicans are racist, but so are Democrats and Obama did actually win twice so somebody must have voted for him.

Rather, Obama's election was the just the last time that the New Deal coalition was strong enough to hold together, having deteriorated over the course of the 90s and 2000s because it had no clear purpose. Capital no longer feels any need to throw scraps (like limited NLRB-disciplined union organizing, Social Security, Medicare, consumer surplus) to workers to prevent 1918 happening here, which had been the Democratic Party's entire purpose since FDR. It had seemed increasingly ludicrous that revolution would come to America over the course of midcentury, but the collapse of the Soviet Union in 1991 brought the farce to light.

In other words, we're still eating the leftovers of anticommunism, the 20th Century's original sin (to mix a metaphor).