The ultimate reason is that if you are poor, the proceeds of crime (theft, burglary, robbery etc) are comparatively more meaningful than to somebody who is wealthy, while the cost of getting caught is comparatively less. A rich professional does not steal loaves of bread to feed their family, because they don't need to and they risk losing their entire livelihood if they do. If you're poor, unemployed and your kids are hungry, the risk/reward calculus is very different.