The reason this game has lost so many subs is because of the sheer amount of MMOs out right now. None of them are bad but, none of them are great either, and it's spreading the populations among all of them pretty thin. No game out right now has a monster amount of subs.
We also have to contend with the "new" factor and the small attention span gamers have due to how games are marketed.
Countless games my friends have told me are going to be great and they hop to them, only to see them leave in droves for the "next great thing" that is just the same thing over again. I actually put my foot down on this matter and told them I will not be wasting money on games I just played with a different title.
You can blame the software companies for this, but that is because they know the gaming community is filled with enough people dumb enough to keep falling for this trap.