Bush put a stake in it by apparently establishing the rule (perhaps from his time as a CIA spook) never admit to who you really are and what you really want. Back in the day there were lefty republicans. Rockefeller was a liberal republican, everyone knew it and he didn't shy away from it and there were a well known group of them that admitted it too. Before Bush I there were people like that, out of the closet GOP lefties. after Bush I there weren't. Oh, there were but they all said they were rock ribbed severe conservatives and never meant a word of it.
Just look at who got nominated after Bush 1: Dole. Bush 2. McCain. Romney. It's pretty obvious who runs the 'conservative' party and what they stand for.