Here are a few reasons:
1.) Cancer is big business, developing cures, research, etc makes a lot of money
2.) The ciggs that cause most of the cancer are even bigger money. I heard a statistic once that in the UK more money is brought into the national health service (health care is free over here) through taxation of ciggs than they actually spend curing it. Also guess where the big plantations and companies are located, in the countries like USA with power to force ciggs to be legal no matter the health risk.
3.) Alcohol just because it is culturally excepted in western society. In many cultures tripping on mushrooms or smoking weed are culturally excepted and alcohol frowned upon. I think it was the Mayans (sorry 'bout spelling) who use to execute children found drunk to teach a lesson to the others.
4.) Plain, simple ignorance; most people don't know what their talking about, myths, inaccuracies, and prejudice prevent people from excepting the true facts. Allot of people simple don't understand about drugs. They have it so twisted in their mind they can't see the forest for the trees.
5.) Hemp was seen as a possible threat to paper mills as it could be used to make paper. The mills + news papers where widely to blame for the pressure for the criminillization of weed in USA so far as I know. Once USA made it illegal allot of pressure is then applied to their allies to follow suit.
6.) Just look where most of the worlds coke comes from, small often communist countries, dictatorships, etc, etc, Columbian drug lords. Most western countries simply don't won't money going into these countries, possibly buying weapons, power, influence, they want to keep them poor and stop potential threats. Think what it would be like if the USA was the no.1 coke producer.....
7.) look where most heroin comes from then read 6
8.) The press can quickly whip up public mass hysteria, when ill informed people start spreading rumors the real point gets lost very soon.
9.) there are so many more but I can't be bothered..........................................................................