I think it's the fact that RE has become a multiplayer brand over two games, rather than 7 million finding the game mechanics "deep" or the soap opera compelling (it's kind of too esoteric in terms of continuity and also that special brand of anime). It's had a nice price point for years too; 20 bucks for 36+ hours of multiplayer shooting is hard to beat. Compared to Uncharted, a game series I loathe, RE6's story is a complete joke in terms of presentation and writing. Same as RE5 and RE4.
But then again, people apparently love the RE movies. They "print money."
But then again, people apparently love the RE movies. They "print money."