It is legal because the tax revenues are huge. The fact is - and this is well documented - that alcohol and tobacco are legal and they cause more harm financially, emotionally, and physically than ALL ILLEGAL DRUGS combined.
In contrast, they will not legalize pot because hemp (which has no drug value) can replace trees, for cotton, and building materials, as well as replace petroleum to make plastic products and thousands of other products. There are major government lobbyists for petroleum, textiles, forestry and many others that would be hurt by this.
Also, the pharmaceutical industry would be greatly harmed, since pot can be used as a pain killer, appetite stimulant, and to treat other ailments. After a while, people would learn to grow their own and NO government agency would see a dime of tax revenue.
Ask any high school kid which is easier to get, pot or alcohol, and they will say pot. Why? because alcohol is regulated, pot and all those other "bad" drugs are not.
The real answer to your question: By government standards, BOTH alcohol and tobacco should be illegal because they meet the criteria for illegal drugs. They are harmful and have no medicinal value.
I've researched this for some years