Mental inertia is a powerful enough force on its own.
The "continental drift" story could also be added to the list of examples.
I think most intelligent people are aware that dietary science is a "dark art," and that cleaving enthusiastically to the "fad of the week" isn't a good idea.
Best dietary advice is generally, "moderation in all things." For the most part, what foods are healthier to eat than others is fairly easy to figure out from simple common sense.