Sean Payton realized something recently. He was forced to give up play calling because of a knee injury he sustained while on the sidelines. Listen to what he realized after this turn of events…
Jimmy Johnson talks with Sean Payton
If Jason Garrett continues to call plays and it affects his game management then he’s going to unsuccessful and eventually fired. And I am not saying some guys can’t do it all because Payton did it all and has been very successful, but also admits that it does take away from some other areas.
Maybe it’s an ego thing.
DCFanatic's Blog 
Easier for him to give them up now. He’s won a SB, won division several times. A coordinator turned HC may not want to lose his job because of someone else running his side of the ball. I didn’t think Red J was a real good OC to begin with. Bunches the yards but always seems to struggle in the Red Zone, not nearly as efficient as we should or need to be. Killed me for years watching us march downfield, then pulls a strange play for negative yards, such as Barber wide for -4. So I can understand wanting to keep that under direct control. That doesn’t excuse him of faulty use of TO’s at the end of a game. I’ve called them when I felt they were the right time but also on occasion so that I could think things over or to allow the team to regroup, beyond true situational need.