Simple - it's in the USA's interests to have a large prison population because prisoners can be put to work making things and be paid a few cents for doing so, which allows the USA to compete with countries where labour costs are far lower.

the prison complex....police lawyers judges builders guards food suppliers uniforms surveillance parole officers etc etc

One of the main drivers for our huge prison population is the war on drugs. Many people are serving prison time for possession charges and other drug related charges.
The war on drugs has been judged a failure but that has not stopped Jeff Sessions from wanting to imprison even more people for it. Maybe this is due to the investments he has in private prisons.

China has the highest number of executions of convicts each year, but is it high enough to bring the prison population down significantly?

Living conditions in US jails are too cushy, that it is an insufficient deterrent?

US laws and sentencing policy see prison as the main way to punish criminals and the principal way to prevent crimes being committed?

High income, technological advancement and draconian legal and penal system not enough to create a society that abides by its laws? A society that has the knowledge (so they know the laws) and discipline not to commit crimes.

Money, politics, racism, class division, do you want me to continue?