The main reason for this is probably due to the fact that within the last three years, the artists at Pixar have wanted to start using off-the-shelf software for modelling, texturing, animating and compositing, instead of in-house tools as previously.
So now they're using Maya, Modo, Mari and Nuke internally, and these apps need to have support for subd to match what PRMan will output.
Is the Disney division using significantly different tools from Pixar? The two were entirely separate companies until the merger and an upheaval in the workflow that significant is not usually so easy to manage.
So now they're using Maya, Modo, Mari and Nuke internally, and these apps need to have support for subd to match what PRMan will output.