Question

why herbivores have long small intenstine

Answer 1

Herbivores feed on plants. Cellulose is a major component of the plant cell and it is difficult to digest. So herbivores have a long small intestine.

Answer 2

Herbivores eating grass need a longer small intestine to allow the cellulose to be digested. Whereas Meat is easier to digest, hence carnivores like tiger have shorter small intestine.