> While there is no doubt that this happened, any one has any idea why there was no reverse push from the fat industry?

The "fat" industry is the animal agriculture industry and it has heavily swayed and influenced American dietary guidelines for decades.

Various food industries presented their side of the argument at a second senate hearing in 1977. This meeting resulted in a watered down version of the Dietary Goals, with less emphasis on reducing meat and dairy products. The American Medical Association also protested the McGovern Report, because it said that providing this basic knowledge on what we should eat might interfere with the medical doctor’s right to prescribe, even though doctors then, and now, know nothing about human nutrition. The effects of the McGovern Report were widespread, and as a result, the consumption of meat, eggs, and milk fell, temporarily.

Industries fought back successfully with every means at their disposal, including hiring lobbyists, purchasing medical and nutrition experts, launching huge advertising campaigns, driving the nutrition education of our children with their bias, and funding nutrition research that favored their products. Their success can be measured by the US food availability data, which documents an increase in mean daily total energy intake from 2,057 kcal in 1970, to 2,405 kcal in 1990, and 2,674 kcal in 2008. We eat more oil, meat, and dairy now than when the McGovern Report was published in 1977. The incidence of obesity and type-2 diabetes has both doubled in that same period of time. These figures are undeniable evidence that industry won and Americans lost.

https://www.drmcdougall.com/misc/2012nl/oct/mcgovern.htm

Total animal food consumption declined, as advised by guidelines...

https://pbs.twimg.com/media/DxMoxXqWsAAjlso.jpg:large

Here's a conjecture - "Sugar" is a product. There is an industry and a lobby for that. Fat is not really a single product (meat, edible oil, chocolate etc.), so probably there was no single lobby which could co-ordinate the effort. Now there is a lot of effort from the meat industry to promote particular diets, so that's perhaps a form of "pushback".

The keto/paleo/etc. diet fads almost certainly are supported in part by the meat industry.

FWIW, I don't think low carb diets are fad, they're here to stay.

I agree with the last point though.

There was no push back because it benefitted them.

By villifying fat, they were able to remove naturally occurring fat from whole foods and turn it into new products to sell consumers.

The public didn't stop consuming fat, they continued consuming fat but it was spread across a greater variety of food stuffs.

An obvious example is skim milk, it's practically water. They don't throw the fat away, it goes into other products like cheese, which people who drink skim milk also buy and consume.

Sugar didn't replace fat in the diet, it displaced fat from some foods, then more foods were added to the diet containing the displaced fat. Sugar just joined the party across the board.

Maybe the "industry" is too fragmented? Although the best fat to eat is leaf lard from pigs (the secret to a really good pie crust), the fat we consume comes in a variety of meat products, oils of numerous plants, nuts including avocados, etc.

You do see studies coming up from time to time about the benefits of this oil or that oil (e.g. olive oil). I suspect the science is similarly biased.