I'm going to make a derisive comparison: A parent trying to cooperate with their child (if they can't assert dominance) will in many cases resort to move closer to the childs position, like getting them a candy as a reward if they put their shoes on. It doesn't matter much if the parent is in the right, if there is no way to convince the other party. Then diplomacy on your part is the last resort.
I'm not saying this is the right thing to do, but it does make logical sense.
If the GOP relentlessly moves to the right, and refuses to be bipartisan on anything (even something as basic and frankly obvious as public healthcare), then you're forced to move to the best they can agree on, as someone mentioned earlier: romneycare. I'm in no way knocking the core ideals of: less government spending, sane tax legislation, less hurdles to start a business, more freedom over what to put in your body, etc. But that's not what any of the GOP brass is about, at least not anymore.
It's fucked, but somehow tens of millions of people keep voting to fuck themselves, on basically all issues other than gun control and abortions. As a non-native, it's beyond baffling. But I guess Republican Jesus loves guns, female subservience and anarcho-capitalism.
Basically, Fox News created the Tea Party. Which in turn helped the rise of Breitbart and other, even more insane outlets. It's a howling echo chamber of lies, whose fuel is hate and ultimate outcome is more inequality... which is perhaps the real motive underneath it all.
So that's a series of guesses, but yeah, still baffling.