Because they run an offense built around that QB. Denver completely changed their offense when Lynch came in, he was in the shotgun most snaps, they let him run, they changed the offense to suit him. NE did the same thing, Dallas with Prescott, Philly with Wentz.
When Stanton comes in, trying to lead us to the win, we're in the same offense. Note he did fairly well dumping it off but once he started trying to inevitably take shots downfield he threw 2 picks and had a 3rd caught just out of bounds.
BA has not changed the offense, hopefully he will now.