if you are poor, and you don't like it (and who would) violence is an easy and attractive short cut option to alter the situation. If you are comfortably well off, there is no need to stick a gun in someone's face to get enough cash to buy a big mac. It's too big a risk for too small a reward. Desperate people do desperate things.
Of course, these things are relative. No one in the US is really poor, not by world standards of poverty. But then poverty comes in different shades. The real poverty is in the mind. These people's material needs are mostly met by a generous welfare system, but only at the cost of their psychological wellbeing. Because they do not earn what they spend, there is this chronic ennui, a lack of self worth, a lack of simply caring about anything. Many of them commit crime for kicks. Because they are bored. Such is the burden socialism lays upon its votaries.