A team of volunteers is still a team. Also, WoW private servers run on a reverse-engineered server infrastructure which seems to require much less babysitting than the actual live servers. I'd guess because they:
* Don't have to deal with quite so many edge cases
* Run on hardware an order of magnitude more powerful than was available to the original WoW server devs, meaning that much simpler solutions with far less sharding / IPC / configuration can still scale sufficiently well
* Don't have to deal with quite so many edge cases
* Run on hardware an order of magnitude more powerful than was available to the original WoW server devs, meaning that much simpler solutions with far less sharding / IPC / configuration can still scale sufficiently well