The area I live in has one of the highest obesity rates in the UK. From where I'm sitting, it looks like people's eating habits really haven't adjusted with their circumstances.
At one time, a lot of people round here were working in heavy manual labour. Now, they're not. But their culture, and therefore diet, hasn't radically altered.
It's a very different situation to just telling people to get a grip – you're talking about entire families and social groups eating badly, not people sneaking off to have a bacon sandwich on the way to work. The habits are therefore far more difficult to break.