It was because they marketed to adults and, as teenagers, they desperately wanted to be adults. A lively grandmother in her 80s confided to me that she started smoking at 10 and quit at 40. That means she began smoking in the 1930s when cigarette ads were still focusing on tobacco leaf quality and Turkish flavor. There were no ads for teenagers and certainly none for pre-teens. In fact, the country barely paid any attention to teens.
Conventional wisdom has it that advertising and marketing are responsible for selling tobacco.
The absence of advertising in Finland and, for a while, in Italy showed that cigarettes sell just fine without ads. The etiology of smoking runs deeper into the realm of mystery than legislators and public health officials may care to venture. But that is truly where the allure has its origins. Teenagers aren't responding to ads nearly so much as they are responding to their own demands of adjusting to the tribulations of growing up. They will accept danger but they want belonging and a kind of personal guide, the mystical amulet popular in myth.