Why do some foods have a higher nutrient density than others?

Because they are healthier lol, if you eat something that isnt high in nutrient density that means its lacking whatever nutrients that you need and youre consuming way more calories than you should. I know I sound choppy so my example should help. If you eat a slice of white bread vs a slice of whole wheat bread you're lacking nutrient density vs eating the healthier version that would provide more nutrients for less calories.