Why are there so many people in the US jails?

The US jail population as a proportion of the general national population is the largest in the developed world.

Why?

I can think of three explanations:

Socialism bitter spot

US has too little socialism (welfare etc) to keep the underclass (i.e., the people who do not want to work) happy (as opposed to Europe where you can live on welfare your whole life), but it has too much socialism to force such people to really work. Thus we are at a bitter spot (worst case, as opposed to a sweet spot, the best case) on the socialist scale.

Diversity

In more homogeneous societies the share of people whose abilities place them outside the labor force is smaller (you gotta know at least some probability and statistics to fully appreciate this theory).

Failure of the legal system

Most people cannot afford an attorney that would stay awake during the whole trial, and without a competent defence attorney a conviction is inevitable.