Re: Love to hear your oppinions on this video.
More seriously, I think the reason there weren't any such strikes was that one of the rules they were going with was "no striking".
Also, I'm no expert, but I hear that it's pretty hard to just break a knee with a kick. Hurt a leg, sure, and leg kicks are a common part of arts like muay thai. But the magical "one kick and ur knee iz broken lolz" thing doesn't, as far as I know, really exist. (Otherwise, MMA fights would end really fast when one guy threw a shot to the other's knee.)
For another thing, the danger of throwing a kick there is that you'd be on one leg while in contact with the other person. You might get counter-thrown pretty badly, and lose the fight over the knife.