There is one word to answer your question. "Pirating"

It can, it just greatly decreases the likelihood that it will.  A massive advertising campaign funded by the console owner, bundling, and likely assistance from the console owner to fund a development budget way beyond the costs managable for most third party developers probably helps.

One other point worth mentioning: the original version of MGS3 had fixed camera angles. That was frustrating as heck in open-air forests and jungles -- the Subsistence version, released in 2005 or thereabouts, fixed this with true 3D perspective, but I think this (plus the complex controls) cut into MGS3 sales.

MGS4's controls, by contrast, are a pitch-perfect delight -- you can jump into the game instantly and start having fun, which I'm sure boosted sales.

MGS4 was hyped a lot as the PS3 savior, if I'm not mistaken.

I agree. Somalian pirates bought PS3s in bulk, hoping to trade them for gold. However, nobody wanted the PS3s, so they looked on IGN, and they saw MGS4, and they all bought it. Then went back to stealing oil tankers.

That is why MGS4 outsold MGS3.

Especially since GTA IV was not an exclusive to PS3. GTA: SA was, so more people were going to spend their money on the more sure of the two games. MGS3 isn't anywhere near as accessible as GTA: SA.

yes loyal fans and also the fact that it is one of the highest rated games this generation and probably even the best game this generation! i know a lot will disagree but it is bloody brilliant! and how could you even question if its overtracked when theres 5 million shipped!