I think it comes down to-

1, Ingredients and what is believed that dogs need nutritionally.
The additives, preservatives etc.. are not popular. Meat content not thought to be high/good enough.

2, People's experiences
Dogs have been hyperactive, badly behaved on certain diets and it is attributed to ingredients.

However that said we have two geriatrics who have had mixed diets over the years- the staple of which has always been the foods that everyone says are "crap" and they are robust, healthy and fit 13 year olds. One of them has literally only ever been to the vet once in his life for ill health and that was for a scratched cornea when he ran into brambles. People might not believe that but it's true and I have no intention of changing as a result. Their mother was on the same dog foods and was 19 when she died. Maybe I have been lucky and my dogs have good genetics anyway- they are also exercised a lot and have been their whole lives so I think that is really important too.