Lamar is a better runner and arguably a better passer than Taysom. So if his style can't be successful in the playoffs, why would anybody want to hand the reigns to Taysom?
At the end of the day, Taysom just isn't a good enough passer to be a successful starter in the NFL. He's had ample opportunities to showcase his skills and he has yet to wow anybody with his passing ability. Being a dual threat is what makes him somewhat successful. But defenses will adapt to that and neutralize him if he is back there full time.