Blizzard says that they cannot find a legal way to allow legacy private server exist and not fail to protect their intellectual property.
"Why not just let Nostalrius continue the way it was? The honest answer is, failure to protect against intellectual property infringement would damage Blizzard’s rights. This applies to anything that uses WoW’s IP, including unofficial servers. And while we’ve looked into the possibility – there is not a clear legal path to protect Blizzard’s IP and grant an operating license to a pirate server. "
http://eu.battle.net/wow/en/forum/topic/17611300963 - paragrapth 4, page 1
They also claimed on multiple occasions that the original version of the game does not exist anymore, making it a reason for not launching their own legacy servers. My simple question is why the heck can't they use the Wow version created by the Nostalrius Begins team?