I don't think the source of the problem is particularly hard to figure out.
Parents are more rushed than ever before, leading them to seek out easier food preparation choices. This means more junky food, even if it's not hot dogs and hamburgers. There are far more prepackaged, microwaveable, loaded with preservatives, options out there than ever before.
Finding nutritious options has become harder. Not harder in the sense that they're not there but harder in the sense that it's harder to distinguish between good and bad.
Anecdotally, I only buy peanut butter with no other ingredients so I only use 1-2 specific brands. But when I started this it was extremely difficult to find out which brands were just peanuts without reading the ingredient list of every single one. If I had been less careful or rushing, I would have just picked up the cheapest, best marketed peanut which ended up being filled with salt and a bunch of random chemicals.
I'm sure many of us have read about how the fruits, vegetables and grains that we buy today are bigger and better looking than the ones from 60 years ago but have only a fraction of the nutrient content?
Mounting evidence shows that many of today’s whole foods aren't as packed with vitamins and nutrients as they were 70 years ago, potentially putting people's...
www.nationalgeographic.co.uk
The basic quality of food is lower than in the past and parents have less time than before. This means that even if they're trying to be healthy with their choices, at least not indifferent to health, there's a strong chance they're still buying unhealthy food.
We see the data in WIC kids because we have the data but it's problematic across the entire country.