The reason the cow is given a sacred title or is considered to be sacred, is related to a couple of matters. The first one being, is that cows are purely sattvic beings. They produce for everyone but ask for nothing in return. Thus it would be savage to partake in taking advantage of them and eating them. The second reason is that the indo-aryans saw the cow as essential to their lifestyle, identity, and efficacy in battle against other cultures. Its likely that proto-indo- european migrations actually began out of fear that other cultures and peoples would inevitably possess the cow.