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.
I think you hit the nail on the head. Precedence of the cultural/social issues over economic issues is more than likely the main reason. I think there's more room for compromise on economic issues as opposed to cultural issues. Both parties have their wedge issues. Republicans exploit God, guns and gays to mobilize their base; Democrats tend to use the minimum wage, Social Security/Medicare to get their base out to vote. It's all about getting out the vote, and ultimately, more people are going to get out to vote if they feel passionately enough about an issue. That was the case in 2004 when the Republicans put so many anti-gay marriage amendments on the ballot to increase "evangelical" turnout, and in 2008 Democrats made George W. Bush their punching bag.
The media is also to blame. We really do not have a watchdog media anymore, and the "mainstream media" has turned into partisan talking heads. There really is no impartial, objective reporting in the MSM. Americans tune into their echo chambers to hear the commentators just enforce their world views and ideological principles, not to obtain factual and balanced information. Just my opinion, though.
I agree with the person who posted that most Americans aren't as partisan as the media and politicians in general. It's really cliched but I think most ordinary people don't care whether you have an R or a D after your name as long as you have real solutions that's going to better their lives, you'll get their vote. That's why independents still decide our elections.
But, for better or for worse, I think people who really pay attention to politics and contemporary issues are skewed one way or the other. You almost have to be when your whole world revolves around nothing but the subject.
