Willpower is hard to maintain for extended periods of time, especially if our dietary rules are too strict. So dieting is hard because people haven't learned the difference between willpower and commitment to long-term behaviour change. Diets make you feel hungry and deprived. Research shows that, whether you are fat or thin, diets make you very hungry and create powerful cravings for the very foods that dieters try to stay away from - such as sugars and fats. This kind of thinking is likely to lead to rebellious overeating.
Dieters lapse and collapse.
A diet only works for as long as you are on it. Most people get bored with rigid eating plans and go off the rails from time to time. The trouble is that for many people a lapse is a sign of failure. Such people go from diet to diet hoping to find the one that will stop them from failing, but such a diet doesn't exist, and they may end up bigger than ever each time they try. Diets fail to address the emotional aspect of overeating.