It's the same reason that applies to all other social and political problems -- they don't end because those in power don't want them to end. Take the illegals pouring across the Mexican border. Does anyone really think it would be difficult to stop them coming into the United States? Of course not. The illegals could be stopped completely in a single day. They continue to pour in because those with the power want them to pour in. The same truth applies to the drug problem, the unemployment problem, the "homeless" problem, the crime problem. If those in power wanted to stop them, they could all be stopped overnight.