Mitch had a sack % of 7.5% when he got benched. Since replacing Mitch, Foles has a sack % of only 2.6%. That's the biggest reason why the offense has looked better under Foles. He gets rid of the ball quickly enough to avoid sacks even when the blocking is bad.
Relatedly, one of the problems with quarterback rating is that it rewards players for eating sacks and punishes them for throwing the ball away when there's nothing there, so it doesn't matter that Mitch's QB rating a little higher than Foles and people should stop bringing that up.