It's perplexing. Pres. Trump is a very, very smart man, and historically has been very hard to fool. I'm just speculating here, but I'm wondering if the majority of "unwise" appointments deceived him by being on their best behavior until the appointment, after which they felt free to show their true colors.
It's also possible that like some of us (myself included) were, he was swayed in part by "name recognition".
Bill Barr has a long and well-known history as a DC swamp creature. For crying out loud, he was Daddy Bush's AG. That should've had him tarred with the loser brush from the get go.
Trump's greatest weakness was in his insistence to surround himself with people who never wanted him to be president to begin with. It's as if he had this compulsion to both drain the swamp and be accepted and liked by the swamp as a member.
I just think he believed the higher circles in D.C. worked like a corporation - and the best people had the most name recognition. It took him a while to figure out that people actually gained that recognition by being the best liars.