Any time news of decrease in subscribers for this game comes out, everyone on WoW forums chimes in with their own reasons and they are usually always from their own point of view (naturally). A hardcore raider will tell you its because the game has gotten too easy and casual, a PvPer will tell you that it is because the game is too imbalanced and not enough focus on PvP is the reason, a casual player will tell you it is because the game is too difficult and there arent enough things to do for casuals etc etc.
One of the things that goes beyond game being too easy/too hard/too casual/too hardcore or whatever else players are saying that usually contradicts one another is that World of Warcraft has indoctrinated the players. In the sense that, they can just buy the newest expansion, play for 1-2 months and be done with it, there is no reason to continue playing and working on progression + gear, when a new expansion is coming anyway. Why play right now? Why devote hours to achieve something when in November you will see the next expansion at Blizzcon, making the game right now seem irrelevant. For fun? Well a large fun factor in MMOs is getting better loot and achieving new milestones for yourself, your guild or your arena team. That can be difficult to get motivated for when you know a new expansion is right around the corner.
This happened before in Everquest and many other MMOs before WoW, but for much of the WoW player base it was their first MMO, so it really mattered that you had good gear back then, because people had not witnessed what a *reset* really meant in a MMO. Even if you crave WoW right now, most people would just wait til the next expansion and jump in on the *reset*.
TLDR: There is a diminishing returns in playing WoW (and most MMO's) you get most out of the game just leveling the first month of the expansion and then quitting. Instead of spending months on end farming gear for...no real reason unless you care about raid progression.