The whole premise of private, for-profit, prisons has always seemed deeply troubling to me. The focus of a healthy prison system should be to rehabilitate people and help them turn their lives around so that they contribute positively to society. That can take years of hard work by the individual, and require assistance from professionals (therapists, teachers, ...).
A for-profit company by definition seeks to maximize profits. Expensive rehabilitation is therefore immediately off the table, or given at the minimum allowable level. That is just not in the best interest of neither the individuals in the prison system, nor the rest of society.