There is a very big difference between clinical hypnotherapy and stage hypnotism. While the former is a therapeutic process for the benefit of the client, the latter is a performance, a show designed purely to entertain the audience.
The people who volunteer as subjects for stage hypnotism, are in effect self-selecting themselves, are happy to lose their inhibitions and be the centre of attention (extroverts), and are willing to go along with the show, whether or not they are actually in a state of hypnosis.