The result of WoW's huge success was that everybody and their brother decided to make a MMORPG. This was driven by executives who want to make a lot of money rather than by the creative folks in these companies, and so what ultimately resulted was that the executives wanted to call the shots in the creative process of these games. In their minds WoW was a huge success, so if they made a game which was very similar to WoW, they would have success as well. Everyone wanted those million-subscriber numbers, and those billions in revenue.
In the end, the number of MMORPGs on the market literally doubled - perhaps quadrupled - in about 3-4 years' time. Most of these MMORPG's had very limited success. With 150 WoW clones on the market, it's rather reasonable to expect that the players aren't going to flock to the 43rd WoW clone, rather than the 87th WoW clone. With nothing unique to offer in their game, these companies found themselves with subscriber numbers perhaps in the 50k-100k player range if they made a decent game.