Why cow are sacred in Hinduism?

In Hinduism, a cow (Sanskrit: go) is a source of food (milk) and a symbol of life and should not be killed. Hinduism does not worship cows, but considers them sacred animals for reasons including: 

• Milk, cows as a source of milk which in Hindu life in India are widely used in various religious ceremonies in temples. In ancient Indian tradition, bull and buffalo were sacrificed and the meat was eaten, but not for cows, because the killing of cows as milk producers was prohibited. The holy verses in the Rigveda regard cows as Devi (goddess) which is called Aditi (mother of the gods). In ancient India, it was highly recommended to follow vegetarianism, in the Law of Manu, The Laws of Manu, V / 56 stated that "there is no sin in eating meat ... but abstension brings great rewards". The definition of meat here is of course not cow.
• Helping human work, cows are widely used to help humans cultivate agricultural land, so that cows are not killed for food but as work aids.
• Reincarnation, belief in reincarnation where the soul can be reborn in various forms of life where humans are the highest level of reincarnation.
• Religious, cows appear in many places in Hindu scriptures / literature, each God has their own animal as a vehicle for transportation, Lord Vishnu rides an eagle, Lord Brahma rides a swan, Lord Shiva rides a bull named Nandi. The incarnation of Lord Vishnu, Krishna, was a cow herder and the cows were very obedient to his orders by blowing the flute full of love. In another literary account of the sacred cow called Kamadhenu it is said that its milk was given to Lord Vishnu. 

From the above aspects and added with other beliefs, the tradition continues so that cows remain sacred animals in Indian religion and culture. It is very different from Hinduism in Indonesia, for our Hindu friends in Java, we are very abstinent from eating cows and still hold on to this tradition as is the case with Hinduism in India, but for our Hindu friends in Bali where the majority of the population is Hindu, it is still found that Hindu followers who do not abstain from eating beef, although the number between those who eat beef and abstain from eating beef can be said to be balanced, I have not found how this happened, and there has been no dispute so far between the two groups; Even I who received religious education at school from elementary school to college level has never been touched on this topic,  I will try to find references why this happened in Bali and until now I'm really still curious.