Why must animals be conscious when slaughtered?

Question

EU court rules states can mandate stunning animals for slaughter reports:

Animal rights activists had pushed for the ban which could prevent kosher and halal ritual slaughter which requires livestock to be conscious when their throats are slit.

I know animals must be healthy, but being stunned isn't a disease, so why must they be conscious?

UPDATE:
Most of the comments and answers explain why pre-stunning is not a good idea (both because if under-done it could be less humane, and because if over-done it could have caused an undetectable fatal injury, which would make the meat non-kosher).

These explanations are useful and informative, and certainly I have no disagreement with any of them, but that is not what I'm asking.

The popularly published information isn't that the animal shouldn't be stunned, but that it "must be conscious".

I am asking where this requirement for consciousness phrase comes from.

Answer 1

Consider R. Emanuel Holzer's description of his testimony with R. Joseph B. Soloveitchik to the United States Senate hearing on this matter:

One of the issues of concern at the time was stunning the animals before slaughter so they would not feel pain. I was asked if the animal must be conscious during shechitah. Wishing to defend the position that stunning should not be allowed by halachah, I responded that the animal must be fully conscious at the moment that the shechitah is performed. As soon as I said this, the Rav began tapping his pen on the table. When I looked over, he caught my eye and asked aloud, "tell me Rabbi Holzer, where is the source for that law?"

I was taken aback, but quickly realized that I did not actually have any basis for my statement. I retracted it immediately, and continued with the testimony. After the session was adjourned I approached the Rav and said, "Rebbe, you shechted me in there!"

The Rav smiled at my pun, but explained that when you give testimony, you must have complete honesty and integrity at all times. And what if the cow was sleeping when the shochet approached? Would it be an invalid shechitah? I was trying to overemphasize certain aspects of halachah in order to maneuver a better position, and the Rav wouldn't have it. He would brook no misrepresentation of halachah, no matter what was at stake.

^{(Mentor of Generations: Reflections on Rabbi Joseph B. Soloveitchik p. 285-286)}

Answer 2

There is no law that requires the animal to be awake. However, the term conscious used here means awake and not manipulated. One of the reasons that stunning is not allowed is that maybe the animal will become a treifa (develop a life-threatening injury) from the stun.

The wording conscious just means alive and without any brain injury, or any halachic injury.