That's also probably because there wasn't fast food joints everywhere back then, and food was usually made from scratch, and junk food wasn't prevalent in supermarkets back then.
There are lots of reasons for everything. But I did see a photo essay recently of “celebrities who smoke”. It was overwhelmingly a list of very thin Hollywood women. They know what cigarettes do for them. They smoke with a purpose. Count on it.
Plus sugar wasn’t so subsidized by the government making it cheap and plentiful. And high-fructose corn syrup had not yet been invented.