Question

the cavecum is well developed in herbivores but not in man. why

Answer 1

cause humans are omnivores.......

Answer 2

These are actually a wider broader and flatter adapted version of molar tooth. In case of herbivores, these are needed to break the hard plant parts like bark or seeds.

But humans are omnivores. As humans have an other option as food which are flesh or meat, from the starting of evolution, they avoided too hard plant parts(like bark or seeds) and thus, their molar didn't evolve as herbivores.