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.
Of course, if that particular little nugget of wootness wasn't already enough to get people excited... Guild banks! How long have people wanted that functionality in the program? Yeah. I don't know about the rest of you guys, but that's a whole bunch of spots that a guild bank could clear in my bank bags. Finally a place to store all those hard to get crafting materials that many guilds accumulate over time. And to do that without people having to give up character slots to create bank alts. Obviously, at this stage of the game, nobody other than Blizzard has the vaguest idea how they'll choose to implement guild banks, but just knowing that they're out there on the horizon is at least with the happy-dance making.
Now if the Devs can just get the different class troubles worked out...