Because he takes away minutes from players that need to develop. Whether he is good, mediocre/same, or sucks. It takes away time from Kanter, Favors, Gobert, Hood.
If he sucks he won't get minutes. If he makes the team better and helps win games, he'll play more minutes, and they are deserved.
You'd prefer 6 Ian Clarks to round out the roster? Should the Jazz even draft someone next year? They've gotta be careful to not add too much depth and versatility