Because state owned prisons do not sign contracts with the state requiring % full or penalties. Only private prisons sign contracts with governments requiring 9

Because state owned prisons do not sign contracts with the state requiring % full or penalties. Only private prisons sign contracts with governments requiring 90%+ capacity or penalties. Thus only private prisons truly incentivize local governments to incarcerate more people to avoid extra costs, while if that facility is run by the taxpayer, they enjoy the savings of hiring less guards and incarcerating less people.