The key quote, "Incarceration destroys families and jobs, exactly what people need to have in order to stay away from crime." If we had wanted to create a permanent underclass, replacing corporal punishment with prison would have been an obvious step in the process.
Obviously that's not why people find imprisonment so preferable to torture, though; TheOtherDave's "sacred anti-value" explanation is correct there. It would be interesting to know exactly how a once-common punishment became seen as unambiguously evil, though, in the face of "tough on crime" posturing, lengthening prison sentences, etc.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.