I interpreted it as a potential explainer for why this sort of result is unlikely to spread broadly.

Do you have knowledge of, eg of New Hampshire (which is mentioned as a counter example in the article?)

I think NH doesn't have any private prisons either, and hasn't since 2000. Private prisons only have about 10% of the total prison population anyway.

https://www.criminon.org/where-we-work/united-states/new-ham...

https://usafacts.org/articles/how-many-states-use-private-pr...