I've been told the staff wanted to start Graham earlier, but he just didn't know the playbook enough for them to feel comfortable taking over the offense. I think others on here have pretty much said the same thing...
@Ibeeballin I believe is one who has said something similar.
Remember, Graham missed a lot of time due to academics, then he got hurt at the beginning of the season. He didn't have that many snaps under his belt to prove to the coaches he had command of the offense. Graham and the rest of the team haven't even had an entire year in this offense...imagine missing most of the that time restrained because of off the field issues and injury.
Now that he's gotten more snaps, you can tell the offense is running better with him in, and the coaches are giving him a longer leash.