Because its not worth buying for them. The gameplay does not have universal appeal. Game reviews and your personal opinion are not close to generalizable enough to predict which games will or will not sell. The majority of video game players simply have no interest in playing a Metal Gear Solid game.

Because I wait for the extra editon (Existence).

because is a game for hardcore gamers and possibly fan of the series...

Because, whilst being a AAA title, it never had the mass market appeal of say a Halo or Gran Turismo (who are both very different kinds of AAA themselves, but sell well).

Also I'm guessing because of the pick up and play appeal MGS4 doesn't have.

 this here is the answer, the thing is in this game, even if you can do anything you shouldnt, you shopuld do what snake would, and that takes patience. Also is very story driven.

The game isn't for everyone. And it is very story driven.

because the game is too good and we need to play crap game like non metal gears games?

To put it in perspective, MGS2 and MGS3 sold roughly 5.5m and 4m copies on the PS2, and that's the PS2 that has a 100+ million units sold worldwide. So MGS4's attach rate on the PS3 is actually far more impressive than MGS2 and MGS3. And to answer your question, it is pretty far-fectched to expect any game to have an attach rate of 50% on any system. I don't think you'll find more than a few games in existence that even comes close to that kind of attach rate.

Maybe because IGN and gamespot don't matter at all.

@jacks81x
you hit the nail on the head, no game will ever have a 50 % attach rate unless it is bundled( Wii Sports).

ot, mgs has a certain audience.