I think there are two big issues here:
1. During the Bush 43 years, the intellectual basis of conservative ideology collapsed. Tax cuts failed to spur growth, as many conservatives expected (the Heritage Foundation predicted that the cuts would completely eliminate the debt by the end of Bush's term!), and in fact, growth following the Bush cuts was much slower than growth following the Clinton tax hikes. Deregulation/underregulation led to the GFC (not specifically related to Bush, but the general notion that deregulation is an unqualified boon did not survive Bush's presidency). The FP ideas of the movement were put into practice, and the results were equally a disaster. Cultural conservatism was identified with the religious right, which became hugely unfashionable.
If, as a party, your whole pitch to voters, your whole reason for existing, is essentially proved wrong, what can you do? You still hate the other guys. But pretty much all you got is that hate and identity politics (and no longer the nice but hypocritical Christian type--just nasty, racialized identity politics and calling everyone on the other side a criminal).
2. As this is all happening, as all the news coming out is bad, conservative thought leaders are denying reality and then saying that the usual sources of information--gov't-collected stats, academia, the media--are all just biased against them. The movement starts to turn against the idea of objective truth. You have the rise of think tanks as an alternative to academia and the rise of an alternative-reality media sphere. Turns out that telling conservatives, especially older ones, what they want to hear even if it's not true is a hugely lucrative business.
And BTW, with regard to that alternate reality, the real problem isn't gullibility as much as cynicism. On some level, I think most Fox viewers (to say nothing of InfoWars, Breitbart, or Prison Planet consumers) realize that it's kind of a game, but they just think, "we have our comforting lies, and other people have theirs." If someone believes something that's false, you can usually convince them. If someone doesn't believe that anyone really cares about what's true, they're just going to profess whatever beliefs give them more power.