Control. Simple as that. The Drug War is a boon for our increasingly militarized police; plus, you have to fill all that prison space. Those who say, "But drug revenues would help the economy, etc" always overlook this obvious point.
Still, it looks like Obama's DOJ may pull back on the drug bust front, at least as far as weed goes. But legalize it? In this hypocritically puritan culture? No chance.
Dennis