Everywhere in the Bhagavatam, Lord Krishna keeps asking us to worship cows, not to harm them etc.
Why only cows, why not other animals?
All animals do something or the other for nature. Together they all keep the food cycle alive. Then why give special importance to cows?
Why cow is given special importance is described in Mahabharata: Anusasana Parva: Chapter 50, 51, 76 to 83 in detail.
Here is the English translation of few verses:
Yudhishthira asked Bhishma
पवित्राणां पवित्रं यच छरेष्ठं लॊके च यद भवेत
पावनं परमं चैव तन मे बरूहि पितामह ||1||
Tell me, O grandsire, what is that which is the most sacred of all sacred things in the world, other than that which has been already mentioned, and which is the highest of all sanctifying objects.'
Then, Bhishma replied
गावॊ महार्थाः पुण्याश च तारयन्ति च मानवान
धारयन्ति परजाश चेमाः पयसा हविषा तथा ||2||
Kine are the foremost of all objects. They are highly sacred and they rescue men (from all kinds of sin and distress). With their milk and with the Havi manufactured therefrom, kine uphold all creatures in the universe.
न हि पुण्यतमं किं चिद गॊभ्यॊ भरतसत्तम
एताः पवित्राः पुण्याश च तरिषु लॊकेष्व अनुत्तमाः
देवानाम उपरिष्टाच च गावः परतिवसन्ति वै ||3, 4.1||
O best of the Bharatas, there is nothing that is more sacred than kine. The foremost of all things in the three worlds, kine are themselves sacred and capable of cleansing others.
Kine reside in a region that is even higher than the region of the deities.
As per rebirth rules, the vegetarian animals will be having human birth sooner than other animals or trees. May be that could be the reason. Also in Bhagavata, earth goddess was in cows form when going to talk with Brahma Deva. Raja Pareekshith saw the earth goddess as a cow.
Also there can be other reasons too. Cows are calm in their nature and usually those animals do not try to hurt any others. Indian culture was deeply associated with agriculture and cattle are an essential part of farming. So cow gave a way of living for the society in ancient India. Hinduism is a culture of truth and it always stick to the truthfulness and rightness. So the people in that culture can not simply forget and avoid anything that have done something useful for them in the past. It is the Dharma and that is why it is known as 'Sanathana Dharma'. So it is not just the economics playing the role here, it is Dharma.
Other animals too are considered to be protected as per Dharma. Ahimsa Paramo Dharma it is a basic block of Hinduism.
I think you answered your own question. Because the sacred book, i.e., Bhagavatam says so.
That's most likely to be the answer as why people who practice Hinduism does it.
On the other hand if you're really interested in why would any animal would be considered sacred in a particular (sub)culture, then the economic/utilitarian argument together with the formalization of such truths into some system of beliefs would be closer to the truth.
Sacred cow, Hindus worship a number of other animals such as snakes, elephants and monkeys besides cows. However cows are considered the most sacred as it is believed that more than 3 crore Hindu gods and goddesses reside in a cow's body. The cow is considered sacred and called go-mata(cow-mother) since it provides milk and is a harmless animal. Dairy products are important in several Hindu rituals such as yajnas where ghee (clarified butter) is used as offerings to gods. In olden times when the cows grazed on medicinal plants on hills, the milk and urine of cows had medicinal values. Thus, go-mutra(urine) was used to treat diseases. For all these reasons , the cow is the most sacred animal for Hindus and men who owned most cows were considered richest( go-maan).
I can think of two main reasons as to why the Cows are accorded high importance amongst all the animals:
Cows have manifested from god Viṣṇu and thereby, as such, are accorded the highest status amongst all animals.
Further, as per Scriptural injunctions, of all animals, only the products (like clarified butter, urine, dung) obtained from Cow and its milk are eligible for sacrifice and other sacred works, and thus Cows are accorded the highest status.
Ṛṣi Mārkaṇḍeya says in the Skanda Purāṇa Verse 5.3.83.112
श्रीमार्कण्डेय उवाच ।
सर्वदेवमयो विष्णुर्गावो विष्णुशरीरजाः ।
देवास्तदुभयात्तस्मात्कल्पिता विविधा जनैः ॥ ११२ ॥Śrī Mārkaṇḍeya said:
- Viṣṇu is identical with all the Devas. Cows are born of Viṣṇu’s body. On account of these two things, the different Devas are attributed by the people (to the different limbs of Cow).
And Śrīmad-Bhāgavatam Purāṇa says -
मूलं हि विष्णुर्देवानां यत्र धर्म: सनातन: । तस्य च ब्रह्म गोविप्रास्तपो यज्ञा: सदक्षिणा: ॥ ३९ ॥
- Visnu is the root (the mainstay) of all gods. Where there's Visnu there abides the eternal righteousness (dharma). The roots of the dharma are the Vedas, COWS, Brāhmiṇas, asceticism, and sacrifices, with donations of liberal gifts.
तस्मात् सर्वात्मना राजन् ब्राह्मणान् ब्रह्मवादिन: । तपस्विनो यज्ञशीलान् गाश्च हन्मो हविर्दुघा: ॥ ४० ॥
- Therefore, Oh King, we shall kill by all (possible) means Brahmanas, the reciters of the Vedas, the ascetics performing penance and those continually engaged in sacrificial acts, and cows yielding all the materials for sacrificial oblations.
विप्रा गावश्च वेदाश्च तप: सत्यं दम: शम: । श्रद्धा दया तितिक्षा च क्रतवश्च हरेस्तनू: ॥ ४१ ॥
- Brāhmaṇas, cows, the Vedas, penance, truthfulness, control of senses, the serenity of mind, faith, mercy, endurance, and the sacrifices constitute the body of Hari.
English Translation by Motilal Banarsidass Publications
All animals are given importance. But how can you possibly reference each specific life form in one place. So cows are used as a representative of all animals. Most rituals in Hinduism are representative of something larger in general.
Cows are specifically worshiped as an example because of the benefit they provide. In India, milk is a MAJOR source of nutrients. And India is an extremely agricultural country so the cows fertilizer is necessary to sustenance as well. These, among many other reasons, make a cow the MVP of animals.
