I don't know either. rana's author also doesn't know, but had this to say:

"I'm not expert at all on cryptography, what I read was GPU is not as efficient calculating EC keys as sha256 hashing, but I'm open to review PR for it and see how it works"

https://github.com/grunch/rana/issues/31#issuecomment-1428147938

Someone else: "This I'm really not sure. Elliptic curves are a lot more complicated than hashing."

https://github.com/grunch/rana/pull/5#issuecomment-1326599577