Nutrition scientists and doctors are working with the human body and its surrounding environment, an exceedingly complex model. They publish their findings in journals with cautions and caveats and data.
A mainstream journalist with traffic quotas or column inches to fill on a slow news day then skims these journals for the next "X makes you fat" or "Y gives you cancer" headline without bothering to read the fine print.
It starts, of course, with Dr. Ancel keys, but he wasn't the only person who signed off on the consensus that a high-carbohydrate diet was the right model for the American people.
I use the scare quotes, because what took place after that, was probably the furthest thing from science you could imagine - presuming we agree that the objective of science isn't to pile on the latest theory, but instead to strengthen those theories, by trying to disprove them.
In the current funding climate and the well known publish(fast and crazy)-or-perish 'incentives', I find it hard to find fault with the bar set for scientists. (Disclosure: I'm one. Although not in nutrition science.)