I think it’s because the science of diet and health has contradicted itself so many times, and has given so many people such bad advice, and has frankly been so unreliable and untrustworthy for so long, that people’s bullshit detectors are less well tuned.

Edit: thinking about it, there’s probably another aspect, linked to why so many people fall for obvious financial scams and obviously bogus “job offers”. It’s because they want to believe it’s real.

Money, like health, is something most people care about. Too good to be true promises about an easy way to health or wealth have a natural appeal, especially to people who don’t have health or wealth and are desperate to improve their lot. And when you really, really want to be healthy again, and science isn’t offering you an easy way to get healthy, it’s easy to convince yourself that horse dewormer will fix all your problems.

Compare that to other types of conspiratorial thinking, like Holocaust denial, or creation science, or flat earth beliefs. For most people, history and physics are of only academic interest. Most people don’t have an emotional commitment to the shape of the Earth. So if they think about it at all, they review what they know and what they’ve been taught without any particular bias and come to the obvious conclusion that the earth isn’t flat.

And when conspiracy theories go mainstream, it’s usually because a large number of people passionately want them to be true.

So it’s not just that the science of diet and health is particularly bad, even though it is. It’s that there’s a particularly large number of people who want an easy, quick solution to good health and turn their brains off when someone promises them what they want.