‘Trump’s not doing what I think he ought to so I’m going to throw a hissy fit!! WAAAAAAAAAHHHHHHH!!!!!!’
A couple of other advantages
He didn’t run in the primary so he has more of a clean slate with supporters of the other candidates.
The Turtle and Ryan can now have fantasies about impeaching Trump after the election so they(and their big donors) are more likely to support the ticket. It isn’t going to happen but they can still delude themselves.
Rand would have been a more interesting choice.