Because just as we have manipulated the appearance of dogs to create different breeds, their dietary needs have been affected as well. For example, chihuahuas were bred to be companions to Aztec royalty and the majority of their diet was vegetables. I feed Titus a very high-pro red meat diet because mastiffs were originally bred for the battlefield and lived a good deal on human and animal remains. He is in great health on it but I know labrador owners whose dogs can't keep it down.
Dogs are animals which have adapted to domestication and altered diets for thousands of years, in many vastly different cultures with likewise vastly different diets. What they thrive on will vary greatly. There are many domesticated breeds which were bred to live on a very high-protein diet, such as dogs originally bred for war or hunting. That's why I stress researching your breed and its needs specifically.