Diet is behavioral. The people who successfully change their diet have to radically change their relationship with food on a deeper level than a lot of people think. For instance many processed, high carb foods are so nutrient poor that simply reducing to a healthy intake number of carbs means that now you're so nutrient poor that your body screams for more. It can even send people to the hospital if you stay in that state long enough. You really have to put your frame of reference for nutrition down and start fresh.
And tell me, what percent of people successfully pull this off conventionally, in the long term, again?
If you took that success percentage and applied it to literally anything else, would it still make sense to blame the person who's trying but failing to be part of that percentage?