Because all the data shows that he's the obvious choice. He destroys every other option in public polling. It's not even close. The only version of reality where Carney isn't the new leader is one where every single Liberal party member collectively suffers a brain aneurysm before the leadership vote.
So? It's not like that's a challenge. All they have to do is parachute him into a safe seat. Given his overall popularity almost any Lib stronghold would vote him in in a heartbeat.