I think Extra Credits covered this rather well a while back, but it's mainly because most MMOs take the form of RPGs, and that market is pretty much dominated by World of Warcraft. People have poured years of their lives into that game, and they're not going to leave it just because something shiner comes along and takes their fancy. The thing is though, there's not anything shinier. Sure the graphics of new MMOs may be better, but that's about it. For the most part they're simple WoW clones, but without the decade of development and infinite resources of Blizzard-Activision backing them.
Of all the MMOs to come along though, I will admit that TOR has the best odds of - well, not beating WoW, but surviving and doing well in spite of it. So long as the game is different enough, and interesting enough, then it may just pull it off.
WoW killers are basically myths though, and the term was pretty much coined to increase the hype surrounding new releases. The reality is that no game likely ever kill WoW, but the playerbase may drop off after time, and Blizzard may start shifting its focus to Titan. WoW will most likely never be killed, but it will eventually shrink and eventually die.
That having been said, it's not going to happen for a long, long while yet.