Most variation in height comes from leg length. So a tall person (I'm thinking 6'5"+) with long femurs and a relatively short torso (compared to the femur) will have to lean the trunk extremely far forward when back squatting in order to keep the load over the center of gravity. In order to squat correctly, the knees will also have to go way over the toes. This is usually limited by flexibility of the ankle joint, and results in sore knees and sore backs.
Placing the load on the front of the shoulders, as in a front squat, is advantageous because it eliminates the extreme torque on the lumbar spine that is seen with the back squat in taller athletes.