The drug laws are designed to be a money maker for local police departments.
Readings - The Drug War s Hidden Economic Agenda Snitch FRONTLINE PBS
Marijuana prohibition, alone, is responsible for creating thousands of jobs in the law-enforcement category, from police personnel, to judges, court personnel, prosecutors, and prison guards. And there are lawyers who specialize in defending marijuana cases. And marijuana laws largely responsible for the success of the private prison business -- which happens to be the only remaining growth industry in America.
During the past two decades a number of medical benefits attributed to marijuana represent a significant threat to the bottom line of the pharmaceutical industry, because if marijuana is legalized it won't be long before a number of widely-used, highly profitable prescription drugs, such as tranquilizers, will become obsolete.
So there is a lot of influential opposition to federal legalization of marijuana.