Limbaugh happened. Glenn Beck, Sean Hannity, and Fox News happened. The idea that compromise=treason happened. It became more about appeasing the noise machine than about holding true to one's principles.
The John McCain of early 2000 (before the SC primary) would have punched 2008 McCain in the face for choosing a whackadoodle like Sarah Palin. But to the alt-right, she was akin to a goddess. And the more traction she got, the deeper and deeper the GOP got into that hole.