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.