Mankind did a lot of things for millions of years that we don't do anymore, so I don't see how that's relevant.

That's a little facetious, isn't it? The fact that there is a fad around this diet has nothing to do with it's provenance. It's a fad diet for most people because they adopted it because of the fad and they will drop it just as quickly. That doesn't mean it's a bad diet, just that fashion is fickle.