The reason for the overflowing prisons is the same reason for everything else..people have to stop spinning on their heads and getting wound up untangling the "why" of things, like that offers some solution or rationality. It doesn't matter why.
As a couple of investigators into such things have said to me, forget the Why, and forget the How, you don't need to know those things initially. Because your first hurdle is making people believe it HAPPENED. and that's hard.
The US has turned into the 4th Reich. It happened. it is an oppressive oligarchy ruled by global money interests. It's Government bodies, from the smallest village council to the White House, exists to serve those money interests. Exclusively. Everything, all the irrelevant details of it, can be debated later.
It's like asking for the "why" of any other horrible fundamental shift to totalitarianism in any other time..who cares? It only matters that we know Germany became Nazi, it happened.
Once you grasp the reality, then you can cope with the details.