(Wild speculation) what if it evolved as a mechanism to keep proto-cats near humans? It's clearly quite a successful trait for a species to become associated with humans, maybe cats evolved a taste for fish as they got closer to fishing humans?
Dogs developed new facial muscles and all sorts of other features as they evolved into a companion species. Maybe cats evolved a taste for food they could only get from us?
My assumption as an ignorant person who happens to live with cats is this: tuna contains a lot of the good things cats evolved eating but in a different form. Humans have this same issue. Tuna became something cats like because they were already wired to like the nutrients in the tuna from other sources.
Dogs developed new facial muscles and all sorts of other features as they evolved into a companion species. Maybe cats evolved a taste for food they could only get from us?