It appears that Bidwill and Ossenfort have decided to hire a defensive oriented coach to run the team, and we can only speculate on the reasons for this decision. Personally, I think a lot of it has to do with Murray. I think Kyler wore Kingsbury out and the Cardinals are determined to not let him do it again.
A head coach with a defensive backround is a little more insulated from the interactions with his quarterback and better able to run the whole team. He can hire the best OC possible and aid him in developing Kyler, without making the whole team about Kyler. This way, if Kyler shows improvement over the next two seasons, then great. But if he doesn't you move on.