The fact that MGS2 outsold MGS3 should tell you the reason, MGS2 was the must have game at the time even though the userbase was much smaller than when MGS3 came out. GT3 outsold GT4 by a ton also, there are tons of examples. Bigger userbase means it can support more big games in more genres, doesn't mean it can't support any.

Not to mention you have the hardcore mgs fans who would follow it to whatever platform it would end up on.

MGS4 is a much more accessible game, gameplaywise. It plays much more like a traditional shooter than the previous games.

Also, it was hyped to no end as the best game ever made.

Also the fact that MGS4 is better then MGS3 in every way could be a factor.

More hype
Less competition
1st game in the gen.

I would say MGS fans followed the series with a hardware purchase ME.

it was probaly the biggest system seller other the bluray.

I loved three thought 4 was great but way to many cut scenes more game play less movies

still it was great.

I have to agreee with axumblade. The bundles probably helped quite a bit. As a matter of fact, im planning on getting the MGS4 PS3 80GB model because it is backward compitable.

Yeah like other people said MGS3 released during a busy christmas season against tons of competition

MGS4 was hyped alot more than MGS3 plus the PS2 had so many AAA games, whilst the PS3's picking were rather sparse before MGS4 came out which probably helped out with its sales a good amount.

MGS4 was bundled too which helped out somewhat on sales too. Not sure if MGS3 was bundled or not.

And the last reason why it sold more was because its just that good of a game.

But remember bigger userbase generally means more copies sold of any given game....

Best game this gen, by a mile and a half.