Could reviews have something to do with it?
I know in NA, reviews are seen as a standard for how good a game is. I'm just speculating but maybe 1 week before the release of this game all the reviews were put up. Sitting on a 80 meta right, maybe people said hey, look at the load times, look at the graphics it hasn't changed 1 bit, look at the reviews and word of mouth it's getting for not adding anything to the series. Add all this up with the hardcore audience in NA and realize that people saw that Tekken 6 is nothing special and cancelled there preorders on a large scale.
Combine this reason with the fact that other great games have been released in the same month and understand that people can't afford all these games on release day. Instead, they may have picked up R&C,Forza 3,UC2,Borderlands etc.
Although, maybe the preorder data was a bit overtracked as well. Looking at the above reasons, hmmm there's your answer..
