Why don't they? Are they afraid people might make the same connections between crime and the war on drugs?
Because war sounds better. It makes people think drugs are so bad that they are literally fighting this evil menace. And most people know from past examples that prohibition doesn't work so they had to call it something else to confuse the morons.
Maybe because people connect the word prohibition with the ban of alcohol and since many people were against alcohol prohibition, more people would be against weed prohibition.