I once read it is an instinct for them not to eat dead in the wild as it could be rotten and they could get sick. For this reason even if it is dead if you move it they think it is alive and will strike to kill it again!

There are many people that have managed to train (so to speak) their snakes to eat dead and not moving. I will let them give you advice on how to get it to happen. I was never succesfull at this, so I still feed live.