Christie is just nasty.
Trump obviously leaned on those in politics he knew best, such as Manafort, who was a longtime tenant in Trump tower, and Christie, who was the governor of a state where Trump had casinos. But as the campaign went on his options improved, Christie looked worse out of Bridgegate, and his SIL proved more valuable and involved than he could have anticipated.
Trump isn’t afraid to fix things he hasn’t got quite right in their first iteration—which is a good trait.