Question

Why did the caecum is well developed in herbivorous but not in man why

Answer 1

Caesum is used to digest cellulose. 
But humans do not need cellulose in the diet.
So Caesum was not required.

Answer 2

The caecum is well developed in ‘herbivorous’ but not in man because a large tube like structure that receive undigested material from small intestine is called caecum.

It is present in large intestine. In small herbivorous such as goat and rabbit, caecum is enlarged and contain bacteria which helps in the digestion of plants and digest cellulose.

Humans do not eat plants in a raw way directly and do not need cellulose. So, in human beings caecum is not well developed.