Question

why do grass eating herbivorous lack canines

Answer 1

Canines are meant for cutting and tearing of the meat.But herbivores don't eat meat.So they are lack in herbivores.
You can see them in dogs,tigers,lions ans other meat eating animals 

Answer 2

Herbivores animals eat grass, vegetables , fruits and many more thing Except Meats or any kind of Flesh. So , they need the Molar  type of teeth to chew or crush the food well . So, they lack canines as they don't need to have Flesh eating things.