It's really pretty simple; the developers tried to have their cake and eat it, too. They built what were ultimately end-game power-grinders that also had a lot of stuff in the game meant to appeal to more casual players. There wasn't enough end-game grinder content to keep those players occupied/satisfied, and the more casual players quickly realized that all of the casual-friendly content was nothing more than a collection of shiny objects meant to divert their attention from the fact that they would never be able to have characters as powerful as the end-game grinders.
You can't keep end-game power-grinders happy without pretty much myopic attention on providing an ongoing stream of new top-end grind content. When all the rest of the content in a game is essentially "run-through" material that is only meant to structure you into the end-game scenario, the casuals see and understand this, and recognize that what they are being used for is to subsidize the development team's focused attention on adding more end-game content.
What developers have failed to realize is that there is no structural reason for either an end-game power-grinder or a casual player to leave WoW or play a non-WoW game that has end-game power-grinding. There is just too much content in WoW for end-game grinders which no new MMOG could hope to match, and as for the casual players, there's no reason to move from one end-game grinder to another and give up their beloved characters that they have invested so much time in.
Why should end-game power-grinders of WoW move to Tera, Rift, Aion or SWToR? Why should players coming into the genre pick any game other than WoW, if they are an end-game power-grinder? Why should anyone risk their valuable time in games that obviously just don't have the staying power or the available content of Wow? Or EQ2, for that matter?
I don't think it would be possible to build a better end-game power-grinder than WoW, so I don't see how any developer could hope to challenge for that market share. No, the real opportunity lies in drawing the casual players from WoW and other games by creating a game that doesn't have end-game power-grinding and offers meaningful, casual player opportunity towards top-end gear and power. After all, it takes casuals much, much longer to consume content; why throw a game out there that simply cannot hope to satisfy the voracious end-game grind appetite of such players, when a casual-focused game provides the dev team with plenty of time to develop new content?