They see keto or paleo and immediately think "as long as I don't eat bread and chips I can have as much bacon, chicken wings, other fried meat as I can eat."  Then they pretty much eat all meat and very little vegetables or fruit.  I used to do paleo. I still use that plan mostly, but I have added some unrefined carbs back to it. It would piss me off so much when I would be talking about diet with someone and inevitbaly a lardass would chime in with "I tried paleo and I gained 40 lbs." Or, "I tried that diet but my cholesterol shot up by 70 points."  Yeah, because you did it wrong fatass. Diets don't fail, only people do.