Private prisons are not paid by the number of people they manage to rehabilitate, or by the living conditions that exist in them. They are paid by the number of prisoners they are holding. They have no incentive to treat these people with respect, and since they are not regulated and are not accountable to any governmental authority, there is nothing that stops them from violating the prisoners' human rights.
It is the private prisons organizations that form the lobby that supports mandatory minimums and other similar draconian measures.