Because it works, quickly.
In traditional psychotherapy, one analyzes the "why's and wherefore's" and while light is shed onto the client's behavior or thoughts and beliefs and an understanding of that behavior/thought/belief is achieved, it doesn't alwayschangethe behavior, thought or belief.
In hypnotherapy, the client's subconscious is being reprogrammed. Think of the subconscious as a computer, it simply puts out what has been programmed into it. Starting from birth until around 8 or 9 years of age, a child is in a state of hypnosis, absorbing and associating information from parents, friends and teachers.