Well MGS sold well due to a lot of factors, large userbase helps but just look at the sales of some Gamecube games to see that even on limited userbase some games can sell like crazy.
The game has a very loyal userbase, that simply bought PS3's primaraly to play this game.
At the time of its release this was (and somewhat still is) the ultimate PS3 exclusive to own, this helped it a lot when PS2 library was incredible, the PS3 library in terms of big exlcusives at the time was well rather limited, less competition = more sales
Intensive bundling and great advertising, MGS 4 ads where everywhere in the UK including the buses.
It had hype and it turned out to be a damn good game, that's why it sold so well.