BO is nearing two years old. The reason why it's maintained it's spot is because of CoD's replayability factor, with one being able to level up 750(?) times. No game has even come close to that number (only counting multiplayer games here). Minecraft, a complete breath of fresh air with superb co-op, is the only game recently released that has that kind of replayability. The reputation it has on the PC doesn't hurt either.
Gotta give props to Mojang for releasing it at such a perfect time though. Many people getting bored of the LastQuarterof2011 games...