I would pose the question, "Why did I start smoking?" After all, I started smoking some 16 years ago, knowing full well there were absolutely no health benefits but MANY negative health risks associated with it. It wasn't even a matter of opinion; I'm nowhere near old enough to have lived during a time where this was even in question. Smoking was and is DEFINITELY bad for you, in no way good, and this wasn't top-secret information.
Of course, without getting into details I know nothing of (social factors, economic factors, gender, race, etc.) a simple answer is, "because I wanted my friends to THINK I was cool." Sure, I can see how stupid that was now, but in the adolescent mind those things don't "click," and there probably lies part of the problem: most people that smoke don't take it up after their teenage years, when the brain still hasn't developed completely, puberty exists, and doing reckless things for, "street cred" is far more valuable than doing things for the long-term benefit of yourself.
You could also ask, "Why do you continue smoking?" Good question. I guess because it's almost ritualistic. I wake up, the first thing I do is grab a cup of coffee and a cigarette. I eat, I want to smoke after. I go on break at work, that break must involve a cigarette with my other smoker peers. I can't explain it in any depth, but maybe that helps. It's one of those things where it shouldn't be there, but my day will feel "off" if it isn't.
In no way am I suggesting anyone should, or that it's cool, or that you should give it a try: you most emphatically should not, it is not, and you should not. But if you want to know why smoking has a good chance of existing in the future, you really only need look to the present: we've known it to be harmful for more than a few decades now, but people have continued to do so, and have even unfortunately recruited new faces. Thankfully, as has been said, smoking (at least in the US) is on a serious decline, possibly due to the advertising regulations and taxes imposed on tobacco. I'd like to think that if I had better friends, Camel Joe didn't make it seem so "cool," and cigarettes cost $6.50 a pack when I was younger, maybe I wouldn't have either.