Quote Originally Posted by MistakeNot View Post
Technical reasons. In addition to all the world servers each datacenter also includes instance servers that handle all the dungeons and other instanced content that potentially involves people from multiple worlds. There are limits to how much load the instance servers can handle, so they try to keep the datacenter populations at a reasonable size to avoid problems - at some earlier expansion releases the instance servers got overloaded and much instability and complaining ensued.
Well, they could add more instance servers. But there's also the servers that store character data, and all world and instance servers within the datacenter need to communicate with those. At some point they'll become a bottleneck. Transferring character data would also be one problem to solve in implementing cross-datacenter instances. There needs to be all kinds of safeguards to prevent losing or duplicating items.