Government is filled with a bunch of squares. DOJ and DEA doubly or triply so.
Did you ever know anybody in school who not only never smoked pot or drank, but thought people who did were human failures? They went on to become professional squares, the US government employs thousands of them, and they've got the power of law enforcement and judicial precedent supporting the careers they've spent their lives building. They also populate the staffs, advisors, and campaign workers of Congresspeople who do smoke and drink.
As much as you think legal weed is a slam dunk derp, there are huge swaths of the country who through religion, politics, bad experiences and/or general upbringing who think it's just as simply a terrible idea.