Rondo is simply not a great one on one player. He has several great skills, and can finish certain types of shots efficiently, but if he doesn't get one of them, he struggles. He is an opportunistic penetrator and only needs a small opening to blow past someone because of his speed, but he is very average at setting up defenders and breaking them down with fakes and crossovers. He needs motion and spacing to create opportunities.
He also seems to need to decide whether to shoot or pass before he gets into the lane.
He just doesn't have the kind of one on one skills that you see with Parker or Chris Paul, who can beat anyone anytime they want.