+When you do that cut in the neck, the blood starts to go out rappidly, and the animal looses conciousness rapidly (while the knife is very sharp, that you he wan't feel much pain first, ever injured your self with a sharp knife or something sharp without knowing? and then you find out later or a minute later when it starts to feel like burning? also, ever felt dissy after a bad injury that you lost some good quantity of blood? ever wondered why many people cut the vains in their hands to suicide? it's so that they loose conciousness when they loose blood or start loosing blood, and then die, let's say "peacefully?" ) , since there's not ennuf blood reaching the brain, so the brain send orders for more blood, so the heart, pumps more blood (that's why you see the animal making those movements after the slaughterin while the blood is going out) , but nothing reachs the brain because of that cut.
Besides that, the blood is a very fertile environement for germs, and it's not very healthy to eat meat full of blood.