Because cruelty toward the defenseless animals is despicable , evil work, inhumane, anyone capable of torturing animals is capable of torturing killing human being.
Defending the voiceless , creating the awareness that animals has life, they feel the pain , suffering, hurts, they have feeling 'that why they capable of loving the human , loyal to those adopt them-
Dogs, cats, other living creatures are creations just like us- we need to defend those unable to defend themselves.
To stop cruelty , reduce the suffering, to live peacefully amongst creations