Ultimately new MMO's all face the same problems that give WoW an advantage: Functionality and Community.
Functionality is the problem that any new MMO has where inevitably THINGS WILL BE BROKEN upon launch. There will be bugs, things will be unbalanced, yadda yadda. This all takes time to fix, and unfortunately we no longer live in a time where it is possible to release a less than perfect product in the MMO market. Why would I buy a game and wait 6 months for the devs to balance it and fix the bugs when I can play WOW as it is?
WOW essentially has the benefit of being pretty much first to market with their product, which has allowed them to establish and fine tune it to a level that other developers haven't been able to match.
Community is the other problem. As others have stated, if you already have established a community of friends in a game, and have invested in that community, and that community wishes to still play, are you seriously going to go off and leave for other pastures? Probably not. Especially if those pastures are not going to be as initially enjoyable as your WoW experience.
Functionality is the problem that any new MMO has where inevitably THINGS WILL BE BROKEN upon launch. There will be bugs, things will be unbalanced, yadda yadda. This all takes time to fix, and unfortunately we no longer live in a time where it is possible to release a less than perfect product in the MMO market. Why would I buy a game and wait 6 months for the devs to balance it and fix the bugs when I can play WOW as it is?
WOW essentially has the benefit of being pretty much first to market with their product, which has allowed them to establish and fine tune it to a level that other developers haven't been able to match.
Community is the other problem. As others have stated, if you already have established a community of friends in a game, and have invested in that community, and that community wishes to still play, are you seriously going to go off and leave for other pastures? Probably not. Especially if those pastures are not going to be as initially enjoyable as your WoW experience.