I think part of it was the fact that it had a delayed release... making the X360 and PC versions seem a lot better. I know they moved up the NA release, but it was slightly botched by some of the dealers in Canada. I.E. if you pre-ordered it you wouldn't get it until the original release date regardless.
The Mass Effect 2 pre-order may have also persuaded people to get either the X360 or PC versions instead of the PS3 version.
At the time PS3 also had a solid line-up with Uncharted 2 and Demon's Souls releasing a week apart in North America. The delay releases put it around the releases of Modern Warfare 2.
It should be selling bucket loads, every store in Canada has it for $39.99 (29.99 for the PC) for boxing week sales.