So why not now? Why not add it in with the Group Finder that they promise us will revolutionize the existing system and replace oQueue? Lore talks about the use of voice chat as a possible element of and advantage to Flex raiding come Warlords, so why not allow players to use a Blizzard option, rather than resorting to TeamSpeak, Skype, Vent, Mumble, Raidcall or others?
Perhaps it is simply because there are so many existing options that take its place. Blizzard can afford to sit idly by and not bother to improve the system because external methods work so well. They have the same attitude with addons in many instances -- why replace something that works just fine? And because the list of options above do work so well, would players really shift away from them to use a system that's been so bad for so long?
The trouble is that many players, not you sitting reading WoW Insider, or readers of other fansites, but many other players don't have a voice option. They're not up on the latest developments, they're not into WoW enough to be here. They don't even raid outside LFR, they don't do anything that requires voice communication. If they have to download, install and configure an external program to get a group, they'll just go elsewhere. And before you say "pffft, it's easy!" remember that there are people out there who won't find it straightforward. A raid team I've been in couldn't use Mumble because half the group couldn't set it up. It was too hard.
But it's not likely to be a high priority for developer time that should really be going into the Group Finder or elsewhere. However, a question -- were that time unlimited, would you have them revamp it? And if they did, would you use it?