Apple’s culture is one of making choices before products get to consumers. You get X or Y, not a choice between X and Y. That’s what makes the existence of iChat (text and video chat) at the same time as iMessages (text chat) and FaceTime (video chat) so… odd.
When Apple launched iChat, it was at a time when compatibility was Apple’s friend. Apple didn’t have the market share to make a proprietary platform play in the space.
Now Apple is the 800 pound gorilla and I’m guessing they want the ability to innovate with their communications tools. Since they don’t control the protocols used by iChat, they can’t make changes they might want to in order to enable new features, architecture changes for mobile, etc.
By creating iMessages and FaceTime they now have their own toolset that they can build on, extend, etc. as they desire. The strength of iOS is really what enables them to do this. They don’t need compatibility anymore.
I expect we’ll see iChat go away in the future. Shipping 2 sets of tools that do the same thing is really not the “Apple way”, and it’s a no-brainer they will favor the tools they’ve just developed and can control.