John Hinderaker of Power Line writes a long column mulling over the party-changing coat-turning of the Cheneys, father and daughter, and people like Bill Kristol, George Will and Mitt Romney. He tries out several hypotheses and isn't totally convinced by any of them.
A hypothesis I find has considerable explanatory power is that of social class; Hinderaker doesn't consider it. A lot of folks find discussions of social class uncomfortable, or even icky.
So how would social class help us understand old line Republicans bailing out of the party? Trump has made a pitch directly to working class America and they buy what he is selling.
Pundits left and right now call the GOP the party of those without college degrees and of those who don't live in large cities. How hard is it to imagine that label creates status anxiety for people with degrees? Not very, in my opinion.
The stalwarts of the old Republican Party often were country club members quite concerned with seeking and maintaining status. Many held what are being called luxury beliefs. Sharing a party affiliation with people of lower social status may feel like "a bridge too" far to them.