The cause is young, feral, black males who refuse to obey the law. Before the war on drugs crime was already rampant in most cities with large black populations. People with no job will have to get money somewhere to buy what they want. Whether drugs are legal or illegal, the criminal element is going to rob people and businesses to get their money.
Like the recent case of the thug in Florida who got himself shot for burglarizing a home. His sister said "how else he goin' to get his money for clothes?"