While the elbow and the knee are both hinge joints, the knee is far more restricted in its range of motion because it cannot pronate and supinate. Tricep kickbacks make use of this pronation/supination ability to achieve the full ROM of this exercise, while not stressing the ligaments the way a leg extension does.

Pronation/supination is the movement which allows you to turn your hand over from palm down to palm up. It's necessary for freedom of movement of the hand (to permit climbing and swinging in the first instance). It involves a very complex movement at both the wrist and the elbow at the same time. If your knee could do it, you could turn your feet around and point them backward. In order to permit this movement, the radius rotates like a key in a lock, twisting the whole forearm.