And don't forget all the religious right freaks whose votes the "get tougher on drugs!" politicians need. "OMG! It's drugs, Harold! Drugs! That evil liberal wants to make drugs legal!" It keeps those uninformed voters happy, and voting republican.

Also, whether it's true or not, or to what degree, I don't know, but I've read some credible arguments that CitiBank is the biggest money launderer in the world. There are large corporate interests who want it kept illegal, because it's good for their profits. Oh, and that would include all the lobbyists for the privatized prison industry. They keep pressure on legislators to keep something as benign as weed illegal because for every person sent to prison, they make money. Same goes for all the mandatory minimums and the "three strikes" laws: there are lobbyists out there who work for big money interests.

Follow the money and you'll usually find the culprit.