There are several theories about why drug related violent crime, which peaked in the late eighties and early nineties and then drastically fell after the mid-nineties into 21st century.
While some claim the drop is due to increased penalties for crack cocaine and the War On Drugs, The Atlantic reports that it is due to lower prices for cocaine and competition from methamphetamine and prescription pills.
The Atlantic reports:
And there’s the missing piece in the DEA’s theory. Once the margin of profit for dealing small amounts of crack cocaine disappeared, being part of the drug trade was no longer worth the persistent threat of violence or the stiff criminal penalties. A 70 percent drop in cocaine prices like the one that occurred in the mid 1990s combined with competition from decentralized sources for methamphetamines and prescription narcotics would completely eliminate the minimum wage drug dealer as a viable profession.
The same goes for turf wars, which Venkatesh saw as the source of the majority of inner-city violence. He saw the life of a drug dealer as relatively violence-free up until territory conflicts with other gangs ensued. Without the high value of cocaine as a commodity, the incentive for protracted gang wars would dwindle as well as eliminate the economy for the illegal weapons, drive-by shootings, and mercenary “warriors” needed to help defend prime dealing locations. Without profit to fight over, Vankatesh thought that “gang violence would likely return to pre-crack levels.”