Looking specifically at RSA and EC algorithms which imply doing operations on integers >= 256 bits (>> 64 bits), I have noticed (from my limited experience) that 99% of the software for crunching big numbers is designed for CPU. For example, there appears to be no actively maintained CUDA package for big-number operations (arbitrary length or even 256-512 bit numbers for EC calculations).
Is there a reason for big number calculations (and thus RSA and EC calculations) to remain on CPU? Is it sub-optimal to run such computations on GPU (I understand that GPUs are not optimized for big number calculations because there is no such requirement in graphics, but can't it be efficiently done anyway)? Will this change in the future or will RSA & EC remain on CPU?