If it's leather it may be outside contaminates as Dave W mentions, but if it's a vinyl wrapped wheel then my bet is that the outer surface of the vinyl has been worn through, and you're into the substrate layers, including glues and resins that bind it together. My old truck got like that - I could clean it up nice, but in a few days it was feeling gummy again, especially in the summer. The only "cure" was to install a cover on the wheel. A nice leather wrap that laces in place might be the ticket? If not, then I'd say you're shopping for an aftermarket wheel, or an OEM/Repop.