Because few people are taking for grant as they do not have heart and have no fear about it. To control these deaf and heartless fellow it becomes very important to pass a rule and punish them if found guilty. After all they are also living beings and they also feel hurt but they can't express their feelings.
Cow is mother not only to her calf but also to entire hamanity.That is why we call her "Gomaatha". Irrespective of caste, creed, sex,religion & age she provides her valuable milk to us. So killing cow is equivalent to killing mother. Killing cow and her clan is sin. So Protect cow.