The M rating is a large draw to many people my age. (15) The average kid seems prefer M rated games over others. I hear plenty of talk about GTA or Halo 2 at our school, and yet equally as good or better games (Ratchet and Clank, GT4, etc.) get dumped upon.
Of course, I think hype has to do with it too. People want to be part of the "in crowd" so they buy into the Halo or GTA hype. Sure, they're both pretty good games, but there are certainly better ones.