Prison is expensive. Housing, feeding, caring for, and reintroducing people to society is an enormous financial burden. Privatizing allows us to push this onto someone else and, in theory, this incentivization for prisons to save money will naturally move them to find the most efficient way to do so.
It’s always a good thing to allow independent contractors to provide resources and services because it allows creativity and, inevitably, improvements to a system that otherwise would have remained consistent. Do you think prisons are doing a terrible job rehabilitating prisoners? Well, open up a prison, you can actually do that, and prove your idea for how to “fix” criminals works better than the current structure of prisons.
Privatizing prisons now makes an inmate a cost. This cost is something a private business will do everything they can to maximize profit on. This includes minimizing healthcare, crowding your prisons, cutting corners wherever possible and, most damningly, pushing a system that encourages prison time to ensure your revenue streams do not dry up. We’ve already seen some private prisons straight paying people for policies that increase incarceration.