I believe the only reason they wouldn't legalize marijuana is because its sooo easy for people to grown themselves. Therefore the government can't gain anything from it. Tobacco companies and liquor are so controlled they get alot of profit from them. Just about anyone with a little research can grow a plant or two or 30 in their house and the government wouldn't know it or be able to gain taxes from it.
I myself believe however, marijuana, alcohol, and cigarettes are all equally addicting. Which people take to as an addiction depends on personality type. I have seen first hand 3 different people each addicted to only one, let it ruin their lives. One lost his son because pot was more important (and he was only doing pot, he was tested), alcoholism killed another, and cigarettes caused cancer and he continued to smoke until the day he died from it. All each depend on the person to their quantity of use and how it effects each person.