Same reason the government won't ban selling cigarettes; they just make too much damn money. Solution: tax the hell out of it and say its bad, but simply reap the profits of having such an addictive drug. Since MJ isn't addictive enough they don't bother wasting their time. They make more money in the long run incarcerating people for drug use than trying to set them free from it. Welcome to the fucked up U.S.A