Private prisons are a conflict of interest. A company that profits on the incarceration of others has the financial incentive to lobby for more restrictive drug laws, minimum-mandatory sentences that take away judicial discretion in sentencing. abolition of parole, "truth-in-sentencing" laws that minimize the possibility of early release for good behavior, and the sort of corruption that leads to inmates being unnecessarily (or even wrongly) infracted questionably for rule violations while confined that lead to loss of good time toward earlier release or a negative mark on their incarceration record that a parole board will see at a parole hearing. To say nothing of Judges being on the take and handing out prison sentences to people who would have ordinarily received probation (e. g. first-time non-violent offenders) in order to use up "bed space", as if prison is some kind of hotel and the Judge is getting a booking fee like Expedia.
Democrats and Republicans alike spread this cancer, but the GOP is far worse, and the industry has far more GOP officials that are pretty much in their pocket.
Bullock's not the worst in this area, and I wouldn't rule him out just because of this. It's possible that the Montana Legislature is pushing this and he has more pressing priorities. This was, however, his chance to be part of the solution, and he passed on it. Private Prisons are a stain and a cancer.