By keeping the level of the graphics slighty behind (at least compared to Half-Life 2 and Doom 3 for the PC), gives the advantage of a greater potential market, meaning that no many people upgrade their PC every 3 months so to play Doom 3 at a super high resolution.
The same applied to the Jedi Knight games, they were always a generation behind in graphics engines, still they are fun to play.
To summarize we want good graphics but no the best available, cause many of us wouldn't be able to play the game.