Pretty uneducated opinion from Gartner (go fig)
If standards for signaling, media, and presence were all that were required, we'd already be an a VoIP paradise. Instead, there's a lot more to voice than just protocols and codecs.
The other problem is that people don't ever want to gut their voice infrastructure (because it's expensive!), so ties to the past hold many enterprises back from fully moving away from the old to the new. The patchwork of voice infrastructure is what integrators are dealing with more than anything. Greenfields aren't that bad, its the so-called brownfields were all the complications arise.