There are several reasons someone might want to avoid mainstream shampoo, like toxins, ineffectiveness, bad quality, damaging, etc. A lot of "high end" soaps/shampoos have ingredients just as bad but maybe add oils and special terms like "natural". Hugo Naturals makes good ones that are mostly plant oils.
Hair cleanses itself naturally and harsh ingredients may strip hair of its natural oils. Many times all that's needed is a cold rinse or washing with some jojoba oil.