Answer:
Because of the fangs. Fangs are meant for gripping food while the other teeth tear. These teeth would be replaced with cutting and grinding teeth for a herbivore.
Step-by-step explanation:
Sharper teeth are for cutting through tough tissues and gripping. Teeth in herbivores are mostly flat with some incisors. The incisors are made for cutting through thick plant tissues and the flat teeth are for grinding these fibers. These teeth are necessary for herbivores because plant matter is harder to digest and therefore needs to be broken down mechanically. This is supported by the fact that herbivores have longer digestive systems than carnivores and omnivores.