Steven Hayward, opining at Power Line on the eternal verities of crime and punishment, and the relevance of basic economics thereto.
The root cause of crime is criminals. Increase the incentives for crime (the result of leniency) and you will get more of it. Like everything else in life, incentives matter.
If anything, our society has an under-incarceration problem. We lock up too few criminals, and those for too-short sentences.