its simple lack of excerise and eating too much,,,,,,you can eat any foods in moderation plan an simple,
Our lifestyles are generally a lot less active than ever before. There is a lot more sitting around doing office work, playing games, and getting entertained.
We also have a culture that centers around the excessive consumption of animal products. And, while they do have a high protein amount, they also have a high fat content. The types of fat in animal products are not the healthy kind needed by humans for healthy brains and hearts, they tend to be unhealthy and fattening.
I'd say the real reason is a combination of those two things.