Reasons given for this decline include free to play games starting to become a major competitor, especially in Asia, as well as content not coming out quick enough to satisfy players. These are certainly plausible, but there could be plenty of other reasons as well. People who got the free one year subscription for pre-ordering Diablo 3 are likely seeing their free subscriptions expiring and taking the opportunity to leave the game. Also, the game is just getting old. It wasn't going to be a 10 million+ subscriber juggernaut forever. Frankly, I'm surprised it still has as many subscribers as it does all these years later.
