Ok. In short, because of intelligence levels. Lower IQ correlates with higher crime. And the painful truth about intelligence is that it is almost entirely genetically defined. As you grow up the genetic factor is smaller, but as you grow older (~adulthood) the gap between genetics and environment narrows. So it is not true that any given person can become smart, if given the right opportunities. This is one of the most pervasive myths about intelligence. And if you stop to think about it open-mindedly, it makes sense. Many observations of separated twins and adopted children led to this finding.

Intelligence can be also denoted in the g factor. It is a metric that defines overall smarts in all disciplines. It is easiest to understand this as overall fitness, for example. A person that competes in the olympic long jump, will also perform above average in other sports. It carries over to other fields, and afaik is the best current metric.

Now it gets controversial if you bring in the human races. There are substantial observed differences in average intelligence. Intelligency levels among a population form a bell curve. When comparing certain racial groups (both when using IQ and g), the difference is more than a whole standard deviation. This difference has long been observed, and because of its controversial nature fallen to the wayside. Ashkenazi jews are the smartest, followed by asians, whites, hispanics and blacks. Look at Silicon Valley for example. There is a reason there are so many asians, as they are most capable in mathematical and geospatial settings. If you look at the racial breakdown of people, they correlate highly with the intelligency bell curves. There are few blacks, more whites, and the most asians.

There is an observed sweet spot of criminality for certain levels of intelligence. When looking at committed crimes this indeed checks out. That sweet spot is at an IQ level that is around the average for blacks. That's why crime increases starting from asians, whites, hispanics and blacks.

Along with that, you've got the welfare state that has effectively destroyed the nuclear family. This is especially true for the black households. Single motherhood is one of the biggest indicators for a life of crime. But it is not true that everyone goes down that path, here also there are some genetic differences that affect the outcome.

Insert the warrior gene (monoamine oxidase A [MAO-A]). People that have this gene are more violent. A less common version of this gene, the two-repeat (2R), is even more strongly linked. Compared to those with a "regular" version of the gene, people with the 2R form are twice more likely to be violent. About 5.5% of blacks possess this gene, while only 0.1% of caucasians and 0.0007% of asians do.

Now, it is important to recognise that the above facts don't mean that anyone belonging to the group in which these facts stack up (MAO-A R2, low IQ, bad neighbourhood) will become a criminal. It is after all, only a propensity. It turns out that good upbringing is most crucial. Abuse at a young age, single motherhood, living in a region with "hood culture" (I don't know how else to word this, glorification of drugs, etc.), ... these kinds of things are the primary triggers for criminal behaviour.

Acknowledging these facts could be a great step towards ending racial tensions. Currently the black man is living under the impression that white racism is causing him to be worse off than other racial groups. The media plays into this idea, because after all "everyone is equal (in terms of intelligence and some other things) right? If someone ends up worse than another, it is due to some form of invisible discrimination." Affirmative action in this sense is a farce, because it tells less intelligent people that they in fact can compete, when in reality they can't. Without the truth this will go on and on. If the truth came out, they wouldn't be able to put the blame outside themselves. They could start to go down the path of self-betterment instead of the wallowing in self-pity.

I hope that gives you a small introduction into the topic. Please, please, please don't fall into the trap of calling me racist. That's a debate stopper, and immediately disqualifies you of having this conversation. If you really think I'm wrong, I'd say prove me wrong with facts, not with random name-calling.