I don't know why it is like that. Robberies, gangs and drugs can be explained by the social fabric in the US, where almost all social mobility has been removed and decent education isn't seen as something beneficial for the society as whole, but rather something you should or shouldn't get dependant on the economic background of your family. As usual people didn't get the point earlier and let their childish labels and name calling get in the way. The study I provided didn't say that blacks are more rapey than white people, actually that comes out pretty even between the two groups. What it did state though was that amongst whites sexual crimes are to a overwhelming degree intraracial, while for blacks it is more often interracial. This can't be fully explained by poverty and social differences such as robbery, gangs, drugs can be.
There are no clear defining reason like you can easily find when looking at the statistics for Norway, where immigrants from MENA countries are vastly overrepresented due to cultural background, with countries with the most antiquated views towards women again being overrepresented within this group (Afghanistan, Iraq and Somalia) when compared to for example Morocco, Tunis, Turkey etc.