I'd say it's because Americans are affraid to death of Russia. Thus it's only natural that games where you can kill russians are so popular. It's sad, but true.
Call of Duty 4 is a great example: it's like the game tries to tell people: "Dont worry! Even if Russia launches its nukes on us our brave soldiers will be able to stop them!!!". It's made like that in games and movies to calm the people down, since they are so affraid.







