"Reagan's presidency marked the start of a long period of skyrocketing rates of incarceration, largely thanks to his unprecedented expansion of the drug war. The number of people behind bars for nonviolent drug law violations increased from 50,000 in 1980 to more than 400,000 by 1997."

Indeed the "drug war" began before the private prison industry, but it has directly and indirectly influenced drug policy to protect and expand its business. https://justicepolicy.org/research/gaming-the-system-how-the...