The U.S. government wanted alcohol illegal, only smuggling was too common and nearly impossible to stop.
You don't see many potheads that are ambitious enough to make underground taverns with automated pot-hiding equipment, so they figure it makes a nice and vulnerable scapegoat.
Plus the government is extremely gullible when it comes to any psychoactive. Try finding how much dirt you can get on salvia divornum and then make an assessment as to whether that drug should be illegal.
and also, it's kind of hard to imagine that the alcohol and cigarette industries aren't all over in D.C. trying desperately to keep it down. The day cannabis becomes legal is the day that alcohol and nicotine each lose a small portion of their sales, and that's a nightmarish scenario that they can't allow.
So everyone but the people who actually know shit about weed gets a voice in its legality. The government is fucking retarded when it comes to pot and people.
but that's just over here, hopefully NZ is smarter haha