The most common accepted answer is some sort of superstimulus theory - modern food is so tasty people find it irresistible. This seems backwards to me in its basic assumption - almost any "traditional" food seems to taste better than almost any "modern" food.

I do tend to enjoy traditional foods more when I eat them, I just don't crave them. For example, I enjoy the taste of homemade Hungarian paprikas more than anything else I eat, but it doesn't instill the cravings I used to get for a Big Mac, coke, and fries. The more we eat certain high calorie foods, the more powerful our cravings seem to become.

I suspect the superstimulus is more from the spike in blood sugar we get from calorically dense processed foods, rather than the super good taste.