The mmorpg industry is an interesting animal. Wow is a giant, most other games are lowly populated (by comparison) and many people question a game's potential longevity before playing (understandable when you will put hundreds of hours into the game). Many games find it hard to carve their niche and make it last, and one even challenges Wow to its own game (Rift). So what makes this industry so interesting?
Go big or go home: most mmos now have small populations, tending tiwards low revenue and even lower new subscription numbers. Mmoers don't like joining a game with a tiny community, and server pop is always an issue. Plus more money means more success as a game (it is a business after all).
Niche-tze: games like this find it hard to appeal to all audiences. Games that try tend to struggle and choose one audience over the rest at a later time. With millions of subscribers you will never please everyone, and a game 'good enough' for everyone will only hold interest for so long.
stability/longevity wow has been going strong for many years and is showing no signs of stopping. Everquest and its sequel are similar, as is ffxi, which some players of ffxiv are flocking back to. Players are investing their time into a game as well as their money. They want to not only be able to play, but also know that their characters and stuff will be available to them for years to come.
Just some thoughts