It's because we love our food to be laced with sugar, particularly high fructose corn syrup. It's found in a lot of foods that wouldn't even seem to call for a sweetener. Controversial concerns about how the body processes HFCS aside, food is simply sweeter and thus higher in caloric content now.
We also eat out more, consuming fewer of our meals at home. Restaurant portions tend to be very generous.
And, in my personal opinion, food is the last vice a lot of people have left that society generally doesn't condemn. You are the devil if you smoke, if you drink too often people start to make quiet assumptions about you, if you engage in casual sex you risk catching a life-long souvenir, but if you eat too much, people won't generally give you as hard a time. After all, it's not like you can give up food completely.