Crowder is a proven vet, has been playing all season, and has taken over a chunk of the minutes of players we traded away (Hood, Johnson) or who are injured (Sefalosha) - O'Neale has taken the rest. And no-one has said his current shooting isn't a concern.
Exum is about to make his third attempt at his rookie season, hasn't proven anything other than he still has potential, hasn't played basketball at all for 6 months, and last played in a meaningful game last June (if you can call the last game of an inevitable playoff sweep "meaningful"). He will also have to take minutes away from active players - which is fine if it's just Burks and Neto, since they are playing themselves out of the rotation anyway, but the amount of minutes he would play would be so small as to be a borderline waste of time. Otherwise he will take minutes away from productive players - again, fine if he's productive, but he has no track record to say he will, other than he looked real good in Summer League. Of course, if that's the version of Dante we get (instead of the Deer-in-Headlights Dante), then that's a great thing. But it still requires more of an adjustment than just slotting Crowder into vacant minutes.