That’s the kind of thing that population health researchers study.

Answers include availability, lack of availability of healthier alternatives, targeted marketing, being missed by anti-smoking campaigns, and peer group influences.

While smoking may be a personal choice, it is not made in a vacuum.

Smoking is “reward now, pay later”

http://journals.plos.org/plosone/article?id=10.1371/journal....

I suspect because if your peer group smoke it is much more difficult to choose to not to smoke?