I'm not so sure though that there was less fat in people's diets years ago. Less junk, yes. My mum was brought up on a typically 1950s diet. Fried breakfasts were common and meals and desserts were often stodgy and high in fat. They were homemade and with plenty of vegetables though. People also walked more.
Why are there more fat people?
I think that people generally have more disposable income. There is more fast food and junk food than ever and it's also cheap! Further compounding the problem is that many people don't have time to cook properly and others don't know how to. Throw in cars and various labour saving appliances, and you're going to get a fatter nation.