Quote Originally Posted by Fhaerron View Post
they don't want to spend the resources
Probably the most likely reason - they'd have to develop and maintain the feature, and with the collection of third-party options that already work just fine (for most) there's no real motivation for them to spend time and money on adding and maintaining the feature.

Honestly, I think the only reason Blizzard even has it for WoW is because they've wanted to turn the Battle.net launcher into a kind of one-stop social spot for players of their games, so adding voice-chat fits within that framework.

There's also the fact that it wasn't actually added to WoW and isn't in the game itself. It's run via the desktop Battle.net app, not the game client.