As someone with a smell allergy, my first approximation to a guess (I am not a chemist), please correct me if I'm mistaken, is something to do with the alcohols contained in most e-cig recipes and borrowed from the perfume industry. Alcohols when aerosolized tend to travel farther than most other smell carriers (that's why perfumes use alcohol) and more directly enter the bloodstream, skipping the body's filters (which makes it particularly useful to e-cigs as a carrier).

I'd suppose other chemicals in traditional cigarettes fill very similar roles.