Priebus shut down any attempts to challenge Trump at the convention. He crushed those little uprisings.
He also had the apparatus in place that Trump used for his campaign as Trump really didn’t have a national organization in place.
Priebus may have been lukewarm on Trump up to the Convention, but from that point onward, he was the best ally President Trump had.
Personally, I think when they realized Trump would win and saw all those millions in small donations, they saw Trump as a money machine and nothing more and then figured, once in office, they could control him or oust him if need be. I know there’s been no shortage of RNC snail mail coming to my mailbox wanting donations.
They see Trump as a cash cow and thought him dumb due to his public persona and lack of refinement in private.