Answer:
I can't say 100% for sure (because I don't work in the vaping industry lol), but I'm guessing it's because teens are generally less educated and more 'bendable' (in a sense) than adults. They don't really know yet the terrible long-term consequences of vaping since they are so young themselves. They are also more susceptible to peer pressure, meaning that even if only a few members of a school start vaping, most of the school could follow suit. This and the natural 'risk-taking' mindset of teenagers make them more likely to try vaping and become addicted...meaning higher profits.
