Which is a large part of the problem when it comes to the scoring system as it is, lots of things made to look objective but are actually subjective. Damage is similar, a judge is not in a position to judge how damaging two similar strikes are.
If we were to look at knockdowns within
@Siver! 's suggestion of being part of the definition of 'downed' we would need a much better definition than them being mentioned in the rules without clarity on what they actually mean within MMA.
Is a leg kick which knocks a guy over a knockdown? If it is how close is that to a leg sweep in some circumstances?