Because Modern Warfare 2 has already been toppled from its perch, after only two weeks on sale. And it's been toppled by Nintendo's multiplayer platformer, incessant TV ads and family appeal winning out over MW2's formula of guns, multiplayer and rubbish story.
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Waaah! Who chose that photo!? I live there! Swindon, Wiltshire, UK :D
It's horrible, never come here.