Because smoking at any level of usage kills, if we know now what we knew when cigarettes were invented they would have always have been banned. Despite everyone knowing smoking kills, people are still too stupid and continue to smoke. Sometimes you have to force people to do what's good for them (e.g wearing seatbelts).
Alcohol does do an extreme amount of damage in some communities however in moderation does not kill & in some studies certain alcoholic drinks have shown to have health benefits (red wine).
For the record I nether drink nor smoke so increasing the tax on alcohol wouldn't bother me.
