[Hnaomyiph]

I think the move wasn’t following fads. In the mid 1900s there was ample evidence pointing at sugar being the culprit for rising obesity rates and other weight-related aliments, but there was a large push from industry to willfully ignore that and label fats as the bad guy. As a result consumers wanted low-fat foods and “healthier” alternatives to traditional fats.

[ParksNet]

Fat goes rancid. Sugar is shelf stable and preservative.

Nutrition is not guided by fads, but by food manufacturer and retailer profit margins.

[mixmastamyk]

A good doc on the subject, reminds one of the tobacco industry a bit earlier:

https://en.wikipedia.org/wiki/Sugar_Coated