It's not like drawing pixels like in a game, but actually calculating volumes and how they interact with light.
It's complex math which is not always suitable for GPU rendering, since GPU's don't use the same logic and instruction set like CPU's and thus are actually very limited compared to CPU's.
GPU's are extremely fast, but "dumb".
Finding algorithms which are suitable for GPU coding + heavily multithreading (since that's mostly what GPU power is based on) is not an easy task.
If you still find this answer not satisfactory, which is perfectly fine with me, I suggest you continue this specific discussion in the appropriate thread.
Search for "cuda" in the "open discussion" section and you'll find the appropriate thread.
Cheers,
Martin