Because the UI work load is longer than anyone wants to think about. We're dealing with mass-appeal items first, like in-game VOIP, Guild banks, Additional base UI, etc.
Now, as I've mentioned elsewhere, the other game that I occasionally cheat on WoW with is EVE Online. (Please, don't throw stuff at me for my blasphemy. It's only once in a while, I promise!) I mention this because EVE just implemented an in-game VOIP a couple of months ago. Let me tell you, having an in-game VOIP capability is seriously sweet. There's no more having to download TeamSpeak because you're grouped up with this guild, or Ventrilo if you're hooked up with that guild, or Skype to talk to someone else you're questing with. (Or maybe like me, you have all three!) For the Mac side of the house, an in-game VOIP could solve all manner of problems -- like the ones I've seen my Hunter officer have with his Mac and a certain professional Vent hosting company's codecs. Bam! Instant compatibility built right into your game. No fuss.