Why would you want a VP who wouldn’t be acceptable as President? The VP is a heartbeat away from the oval office!
How about we send the liberals back over to the traditionally liberal party?
Because you might need him to moderate the presidential candidate's image, and bring in middle-of-the-road voters. That is why Reagan chose Bush rather than some right-winger. Reagan and Bush were never close, and Reagan was meticulous in his political strategizing, based on the copious handwritten notes that were compiled into a book, so this wasn't some offhand decision.