My guess was that once they hired all of the extra people to work on WoW after they realized how much content players kept wanting at such a fast pace, they probably felt that they didn't want to maintain two entirely separate teams for two entirely separate MMOs at the same time.

Hopefully now they will put a majority of those remaining people back on WoW so that we don't have to sit in the final tier of an expansion for an entire year (or more).