Before there was strong for-profit bit in the mix (but it was always there) but the main idea was that rehabilitation is too complicated and doesn't work. Something to the effect of "Oh look we tried, half-hardheartedly, it failed, there is no point wasting time, these people never learn so just lock them up away from the society for as long as possible". That was always going hand in hand with tough-on-crime politics and a general sadistic undercurrent in the American culture that celebrates punishment for punishment's sake.

But once the for profit element was added into the mix there is an actual disincentive to rehabilitate people as it would mean a direct reduction in profits. So before it was bad but this is like adding some gasoline into the fire.

There was for example, the case of a judge in PA who had a deal with the local prison/juvenile center of sorts where he was sending teenagers in for minor infractions and was getting kickbacks.

Prison industrial complex also lobbies the government to keep the War on Drugs going because a reform there also would directly cut profits for them.

There even a whole ecosystem of predators exploiting every single angle possible to milk the prisoners and their families' money and labor. Down to telecom companies with ridiculously expensive charges when prisoners talk to their loved ones, to companies selling food (the commissary) and so on.