I like to think it is the "It's Not Your Fault" culture of today. No one seems to take responsiblity for their own helath.
Also a lack of time to cook. Families where both parents work are at a major disadvantage because of this. I did a report recently that found that families who eat dinner with the family at the table at least 5 times a week have lower risk of obesity (along with many pyscological and health benefits).
They believe this is because of
a) the quality of food
b) conversation slows down the pace that people eat at
c) creates a more stable home enviornment (stress and depression both lead to higher risk of obesity)