Go ahead read it,
"Central Government Act
Article 48 in The Constitution Of India 1949
48. Organisation of agriculture and animal husbandry The State shall endeavour to organise agriculture and animal husbandry on modern and scientific lines and shall, in particular, take steps for preserving and improving the breeds, and prohibiting the slaughter, of cows and calves and other milch and draught cattle."
All milch and draught cattles are covered ,cows are integral part of farmers life, people treat them like family and they are a source of income,(milk,milk products) and has agricultural uses.
Sheep and chicken doesn't have this place in our society, this was identified by our first govt (non BJP) which introduced such laws.