People have a variety of intensities of cravings and capacities for self-regulation (with a magnitude that is largely out of their control). Discipline only works for people where the capacity for self-regulation outweighs the intensity of cravings. These cravings are huge though because our food system is engineered to be both extremely calorie rich and appealing. It's hard to blame people when they're surrounded by fast food restaurants on every corner that has been engineering to target what's called the "bliss point" when experiencing their food.

Looking at this a different way, maybe the discipline / self-regulation needs to be applied at the societal (not individual) level, to improve the environment in which we all live?