Either that many people play it or because it's in the centre of hype. WoW is mainstream because everyone talks about it, because it has made itself over hyped because of commercials.
Same thing for Gears of War, Killzone 2, Fallout 3, Crysis(even though no-one can play it), GTA 4, Tomb raider and if you can't relate to anything previously mentioned here is the mainstream plague that has brought hell on everyone on XBL HALO 3!
Non mainstream games; Jak and daxter, Ratchet and Clank, GRAW, Timeshift, Red Alert, Starcraft etc. etc.