x86_64's win was helped by Intel's Itanium misstep. AMD can't bank on Nvidia making a mistake, and Nvidia seems content with incremental changes to CUDA, contrasted with Intel's 32-bit to 64-bit transition. It is highly unlikely that AMD can find and exploit a similar chink in the amor against CUDA.
If they're content with incremental changes to CUDA then it doesn't cost much to keep updated compatibility and do it as quickly as any users actually adopt changes.
