Out of the ten million WoW players, less than five percent are interested or even aware of end-game content. Most players know that level 70 is the highest level that you can achieve; however, many of them don't know that level 70 is where the most interesting part of WoW begins.
Instead of gaining more experience to level, players work extremely hard to acquire better gear and items. This requires hours of dedication inside dungeons and on raids (a.k.a., much larger dungeons). Raids must be run well by leaders; otherwise, they fail.
Many people just think that end-game content takes too long to get; the end-game is just too hard, takes too much effort.
On top of that, the gear that drops comes in small amounts, meaning only a few members of the party will get anything for all the effort expended in any particular raid. Party members must show up on assigned raid times and follow organized, detailed instructions.
A single mess up from any member of the party can kill the entire group and force everyone to start over. This is just too frustrating for a lot of players, and is even frustrating for existing, hardcore players. Those existing players often call WoW more a job than a game.
Blizzard is toying with the idea of making the end-game easier through The Wrath of the Lich King expansion, but in this writer's opinion anyway, that's a terrible idea. Part of what makes WoW so appealing to those top five percent- the true WoW addicts- is the difficulty.
The best part of pouring all that time and effort into getting the gear is knowing that you're the best of the best. Sure, it sometimes frustrates me that the top level gear is so insanely difficult to get a hold of, but I'd hate it if they made it so anybody could get it.