I'm not sure it was ever about respectability. I think he just didn't have his shit together to actually be president. Campaigns usually start making transition plans long ahead of the election.

The other thing was that he didn't have as much of a stranglehold on the Republican Party in 2016. It was still very possible to be a politician in open opposition to Donald Trump. So when it came time to pick someone, he couldn't pick and choose proven loyalists. He just had to choose from among ranks of people who may or may not be hostile.