Because they need to digest cellulose

Because the digestion of cellulose take a longer time because the enzymes produced by the ruminant bacteria that live in the gut of the hervivores. Longer small intestine ensure that the food stay for a longer duration and proper duration is possible.
Because they have to digest cellulose which need longer small intestine

Digestion of cellulose takes a longer time. Hence, herbivores need a longer small intestine to allow complete digestion of cellulose. Carnivorous animals cannot digest cellulose, hence they have a shorter intestine.