Why introduce all of these Allied Races the expansion after we've dealt with them? Relatively unrelated support to my argument here I know, but the idea of a Witch Doctor being introduced AFTER we've explored what they have to offer isn't unbelievable.
Current/Past/Future aesthetics aside, Witch Doctors sound much better on paper than Necromancers. They both portray relatively similar themes, but Witch Doctors offer qualities that we mostly haven't seen before. Necromancers confliction with Death Knights are far greater than Witch Doctor's conflictions with Warlocks.