Diets go wrong by requiring the dieter to change the type of food eaten. Humans are creatures of habit and usually eat the same foods over and over. Granted, overweight folks are eating too much of the wrong foods. Using a sample week's menu of meals can result in buying unusual ingredients, using a small amount for one recipe, then often wasting the rest. Diets go wrong by making it difficult to prepare food and to eat.
Most diets suggest using fresh foods, cooked from scratch at home.
This requires more meal planning, shopping and preparation time. It's easier and quicker to rely on fast food or convenience foods. The drawback with fast food is the difficulty of controlling exactly what is eaten since the ingredients are not easily known. Even with the improved labeling on convenience foods, there's no guarantee the totals at the end of the day will be within healthy ranges. But trying to eat less and prepare strange new dishes can be discouraging.