[Mister Pickles]

It could be any number of reasons. Diablo III, change of season, lack of interest, lack of development, nonactive staffers and the outright refusal to add more staff members. At this point the majority of the people are simply macroing. Once there's nothing left to macro you can expect some of those players to leave.

You can't really attribute it all to one reason like Diablo III. At this point the Diablo III hype has started to settle. If it were simply Diablo III then some of those players would have returned to the shard. Belgar doesn't count.