I can't claim to know the answer to that one... loyalty, best chance of remaining in the NFL, maybe he didn't mind the constraint (didn't have to be adversarial)... who knows. Now we'll see what Hackett can do with a top QB.
Others have mentioned that his concepts and plays look good. Execution. particularly at the QB position may have been what has been holding him back. Unless something unforeseen happens there will be no excuses after this though