My theory about all of this is that the development team probably got too large too soon and some sort of bloat/scope creep got into the process. The normal response to that is to send people off temporarily to other things while the design core concepts get reset and stuff that was proposed to be added is cut back by a great amount.
That and mobile gaming happened. That was probably totally unforeseen in the beginning. Blizzard getting back into console gaming may have played a part as well.
In any case, it's not hard to imagine that when they started this up during Wrath the game they thought they would release now or next year isn't the right game to release into today's market. Add some or all of that up and it makes perfect sense.
Just a theory though but one based on having worked on large software projects in the past with large development teams. Stuff like this happens.