Unfortunately I really think that it is true that a huge part is due to todays diet.
To sum it up I'd say, the most common reasons why today the Hourglass Figure is on the brink of extinction are:
1) todays diet;
2) fisical exercise as it is done today
3) when girls reach puberty today, they're not stuck into rigid corsets anymore. (Yes, even fashion has to do with it) - wether this third point is good or bad, is not for me to decide.
4) genetical evolution - take a girl (around 20), and compare fotographs of her Great-Grandmother - Grandmother - ****** at around the same age, you will see a progressive change in the figures.