Because alcohol and tobacco producers have already won their battles in the War Against Some Drugs. Big business, don't you know. And they've also proven that Prohibition of their products doesn't work. Whereas Prohibition of cannabis doesn't have any deleterious effects (;P)
"Marijuana, Weed, Cannabis, Dope, Weed, Marijuana,...oh yeah, tobacco and alcohol are the greatest factors in association with later problems, now back to our regularly scheduled program...Marijuana, Weed, Grass, Dope..."