Many are bureaucratic. But I would like to focus on marketing reasons, as I learned them from the researchers that taught the tobacco companies in the first place. You Can't Win by Fighting Your Opponents' War Anti-tobacco groups can afford to advertise, but tobacco companies can afford to advertise more. That means they win, and since smoking is a lure, just bringing up the topic a lot tends to encourage people to try it. Then they get hooked.
Tobacco wins more.
People tend to calculate the odds of calamity in their own favor. Teens often don't calculate at all. If warnings worked, then the blunt messages on cigarette packs would long ago have discouraged our 50 million smokers. No matter what is said on TV, the reality of the 7-Eleven, gas station, supermarket or drug store is that smoking is a staple of our lives. Cigarettes offer you something for your troubles while anti-tobacco messages give nothing.