Nate Silver says that the GOP reluctance to compromise is driven by voter alignments. John Sides has a good post up responding, in which he points to the strong conservativism of GOP activists and politicians as the driving force.
I tend to agree with John, but with a major caveat: I think that only looking at ideology misses a significant piece of the puzzle here. Yes, Republican politicians and activists are very conservative. But they are also, and perhaps more importantly, highly partisan. That's why they can flip positions overnight when the situation calls for it; their positions are informed by the partisan context as much, if not more than, some sort of principles.
But neither of those things is sufficient to explain a party that in many cases appears to do things that undermine its own goals. After all, no matter how conservative Republicans may be, it's still the case that in any situation there's a best (that is, most conservative) deal that can be made, and whether it's partisanship or ideology driving them, they should be trying to make that deal. I'm convinced that the problem is that on top of everything else, the money to be made from conservative extremism introduces perverse incentives that color everything that the GOP does. That, and not ideology or simple partisan polarization, is what I think may be actually scary about politics as it has been practiced for the last several years. Everything about Madisonian politics is based on an assumption that ambition will drive politicians and everyone else in the system to aspire to winning elections. If there are serious incentives to be in the minority, it's not clear that the system will work.
(And I've said it before, so I don't know that I'll write a post about it, but that's part of my answer to Matt Yglesias's argument against the US Constitutional system and in favor of parliamentary government. If the problem with the current system is perverse incentives created by a consumer market exploited by conservative rejectionists, then I'm not sure why parliamentary systems would be the cure. Beyond that, as usual, I'd just point out that the US doesn't have a "presidential" system -- it has a system of separated institutions sharing powers).