To answer your question as to "why"...coaching. Quarterbacks don't develop in a vacuum, or by osmosis. The coach is in charge of personnel, and development. As I have said, it's malfeasance in year 3 to have neither coached up the starters obvious deficiencies, developed a workable backup, nor recruited or transferred in someone you can.
The good news for you and me is that we don't get paid exorbitant amounts of public funds to make, or not make in this case, these types of decisions. That's Pruitt's job...at the moment.