Cows produce, in large quantities, the miracle food, milk. Milk is produced from the blood of the cow, however she is happy to give her blood transformed in a peaceful, non-violent way in the form of milk. The cow is so merciful she is freely giving us her milk which contains all the vitamins, proteins and other nutrients found in flesh. If people drink milk there is absolutely no need for animal killing and no possibility of their diets being deficient in any way. One can survive simply by drinking milk.
Milk is a complete food.
As devotees of Krishna, we have a special interest in protecting cows because Krishna is a cowheard boy. Krishna likes the cows very much. Therefore, because Krishna is very friendly towards the cows, killing cows is more sinful than killing other animals. Any unnecessary killing is sinful so naturally we are vegetarians. But we are more Krishnatarian than vegetarian.