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.