I guess Diminishing returns set in.
I really don't know anyone who quit because they just got bored, most was because of the reasons you guys like to bash, I know 10 people. (Small sample I know) quit because the game was too easy for them.
In the end, it has to be considered a good possibility casualification wasn't reactionary to sub loss. It was an attempt to get even more into the game.
I come with this theory because I remember blizzard basically saying things that elude to that in the ICC Cata era.