He wanted to be president, that's why. He thought he had better ideas and he was right. His chance came 4 years later, but he supported Ford, once Ford was the nominee.
This isn't rocket science.
Reagan beat Ford around the head like a punching bag, and Ford barely held on. If not for Reagan showing Ford to be so weak, Ford could have beat that peanut farmer from Georgia.
The point is this, Reagan ran becasue he was trying to take the party in a directions it desperately needed to go in, even though the majority of GOP right or wrongers could not see it.
I am telling you as sure as I am sitting here that a lurch to the left is going to eviscerate the party. I hope you hope that I and others here are successful in making that not happen.