Question

why do herbivores have longer in small intestine than Carnivores

Answer 1

they mainly take of cellulose
which is a complex sugar
so it needs a longer path for digestion
that's why herbivores have longer small intestine than Carnivores

Answer 2

Herbivores that longer intestines than carnivores to digest grass. These intestines would host many small bacteria that processes and breaks down cellulose into glucose, a source of energy that the host herbivore can use. Without these long intestines, animals such as cows could still eat grass, but would not be able to use the glucose stored in the grass structure.