At least one reason some countries have much higher incarceration rates than others is that they have much higher crime rates than others: http://quod.lib.umich.edu/h/humfig/images/11217607.0002.206-.... The murder rate in the U.S. has been 5-10x higher than in the U.K. since 1900, long before private prisons or the drug war or the other things people blame for crime in the U.S.

The shocking thing about incarceration rates in the U.S. is not just the fact that it quadrupled from 1960 to 1995, it's that even with that increase, the growth in incarceration didn't catch up to the growth in violent crime until about 1998.