The Rise of Partisanship

[All Along The Watchtower]

I'd say the precedence of cultural over economic issues is one reason. Cultural issues seem to divide people like nothing else. Also, the decline of the media into partisan talking points is another reason.

Finally, you have a good deal of moderates and Independents who are indifferent to or turned off by the whole process. That means that partisans on both sides get to carry the debate.