Why is tobacco legal all it brings is problems.
For a real argument, who are some of the richest people in the world (not celebrities), what industries are they in? Tobacco, alcohol, and pharmaceutical industries. If cannabis was legalized it would steer people away from smoking tobacco, tobacco advertisements are aimed towards kids from the ages of 12 to 22. If you introduce a product that can be smoked with very little harmful side effects and gets you high, the 12-22 age group will want that and not tobacco. It is basically the same thing with alcohol. The pharmaceutical industry would take the hardest hit from the legalization of cannabis, cannabionoids are known to be extremly useful for medicince and this has been known for almost 5000 years now, the thing is that people can self medicate taking away lots of demands for medicene.
Also think about all of the government organasations that will be put out of work, the DEA in particualar. Most of the DEA's work comes from busting cannabis deals and drug cartels who deal with cannabis primarily.
Watch the union they have a lot of good information about this.
Also Money=Power, why would the richest people in the world want to give that up.