I think the simple explanation reflects reality the best: the diet in the US and now the rest of the world contains more foods which are rich in sugars and oils, which are highly processed and therefore easily digested, and which were designed to stimulate cravings to facilitate eating and sales.

Rich foods are more calorie dense. Since satiety is partly related to volume of food consumed, rich foods can provide sufficient calories without diminishing hunger.

Highly processed foods are effectively partially pre-digested. This increases the energy availability of the foods. For example, you will get more calories from white flour than from whole wheat flour than from wheat berries.

Finally, products that have been successful are ones that sell more. For food marketers, the goal is to optimize the "bliss point", which is a balance between salt, sugar, and fat content. This is important because, in the past, when people's lives became more sedentary, they ate less in response. Targeting the bliss point has reversed that trend.

Now they can be addictively tasty. And in these busy times with lots of things grabbing our attention, premade foods - even ones which would appear to be made up of all good ingredients - are the easy choice.

To give processed foods a longer shelf life, they need more salt; but to compensate and make them palatable they then need more sugar. So even ignoring the less clear downsides of how processing might affect the chemistry, the simple observation is that foods have more added sugar or sweeteners now than ever before.

In human history, that's a pretty rapid and significant change... much like the body shape changes that have occurred during the same 30 years.