The actual philosophical reason for cow protection is very simple; all living entities deserve protection from slaughter and other violence at the hands of humans. Not only cows, all animals have souls the same as we do. They are all children of Krsna, all dear to Him.
With this view in mind, it can be seen that slaughter is a form of MURDER.
The cow, however, is our MOTHER. Vedic philosophy teaches there are seven mothers:
the birth mother
the wife of the brahmin (if she is not the birth mother)
the wife of the king
the wife of the spiritual master
and the cow.
You may wonder why the cow is one of the seven mothers. It is because she gives her milk to nourish us.