Because they have let one bad defensive day metastasize in their own minds, compounding the problem.
They are going to have to play Bohm at some point against right-handers and Wheeler is less of a groundball pitcher than their other choices so today would have been ideal. And if they are so concerned with defense that just bat Bohm 3 times and put Carargo in for the last 3 innings and the 4th AB if that is what the game situation dictates (i.e. a lead).