The problem is that a lot of those sources of protein aren't just protein. Take legumes; they all have a ton of carbs as well. Nuts? Loaded with fats. (Same with pork belly, TBH; that's a very fatty cut.) On the other hand, boneless, skinless chicken breast has a trivial amount of fat and carbs.

If you were plotting a 3-dimensional chart, factoring in proteins, cost, and other macronutrients, you would likely find that boneless, skinless chicken in general was the highest combination of both purity of protein, and price.