Jason Burns points out, pretty insightfully, on his blog that of all the various options Blizzard lists for quitting the game, simply not having the time to play it isn't one of them. Whenever you decide to cancel your subscription, you get a little survey to fill out, with two lists of options to explain why you're leaving the game. In the past, we've found some pretty silly options in there (and actually, it looks like they've changed quite a bit since we posted about the Will of the Forsaken nerf on the list), but Jason says that as many varied and different reasons as there are for leaving, Blizzard didn't include his: he likes the game, he just doesn't have the time to play it and justify the subscription.
Which is what he eventually typed in. Like so many things with Blizzard, it would be fascinating to see the stats behind the information they're getting here -- do they see a little bump in quitters every time new patch notes come out? Is customer support really the biggest issue people have, or is harassment a major reason for people leaving as well? Just boredom with the game seems like it would be a huge reason to me, but there's not really a clear option for that either.
Unfortunately, we'll probably never know the real stats behind this -- Blizzard isn't going to be revealing why people are leaving their game anytime soon. But it does seem strange that some of the biggest reasons you'd expect aren't on their list of possibles at all.