Why is Accutane use limited if it is so effective?

My son had severe acne and went on his first course of accutane at 15...worked beautifully. Acne came back about eight months later and he went on a second course of accutane which worked again for several months. He is now 17, he completed his second course of accutane about 8 months ago and his acne is returning again. His dermatologist has told us patients can only go on accutane three times. Why is accutane use limited if it is so effective?