Animals that depend solely on animal fleshfor their nutrient requirements are called obligate carnivores while those that also consume non-animal food are called facultative carnivores.
A facultative carnivore does best on a carnivorous diet, but can survive-but-not-thrive on a non-carnivorous one. It prefers not to do that, but when pushed, it will. An omnivore really doesn't care much either way. It eats what's available, vegetable or meat.
It is because dogs prefer carnivorous diet.