I suspect that it is legal fears tbh - it is almost certain that if AMD or anyone else tried to make some kind of CUDA compatibility, nVidia would pretty fiercely sue them into the ground. This is almost certainly why both Intel and AMD bailed on ZLUDA.
One thing I wonder about with AMD is that they know the history of how CUDA got to it's current position, but even if you say trying to compete in that market is fighting yesterday's war and they don't want to dedicate much resources to it they don't seem to have much vision to start and do the long-term commitment to what could be the next big thing. What projects do they have to boost their strengths, and can't be copied easily? The examples I would point to though are Zen (a massive course correction after K10-Bulldozer) and early HSA after acquiring ATi
