because he did not light it up last year with poor OL protection. Confidence and strength in the knee + some OL improvement could change things fast. Also more reps with the guys he will be playing with. Last year it was clear the WRs had a difficult time changing from KJ's passes to CH's passes with more mustard.
I am definitely on the CH bandwagon. I thought POB was really going to push him but if he was still struggling with the playbook I don't think we'll see him. CH does know the offense and he does have the vision to be a great QB.