Modern highly processed food is optimized to our sense of taste, to the extent that they can be called superstimuli. They are also correspondingly unhealthier, on many metrics. (I suppose this is the part in contention... I don't have any sources for this claim, sorry.)

The paleo diet, as well as the Atkins diet and other diets, inadvertently 'works' because highly processed foods tend to be carb-based (crackers, cookies, chips, sugary cereals, sugary yogurts, sugary soft drinks, sugary baked goods), and are thus excluded.