I don't think it's my place to list why there is such a desire for hybridization of fish. I am not a commercial breeder and I definitely don't want to mis represent others. There is plenty of scientific and fish industry information available if one is interested. Similarly for the hybrid vigor discussion, its not my place again but I can speculate that even an 8 cent per fish increase in profit/productivity/ time saving or cost reduction would be hugely significant when you are counting fish by the tonnage and moving stock with pumps. Using my 8 cents savings analogy as a practical example; if fingerings were 10gm each, you would get 1,000,000 fish per 1000kg (one tonne) which would be $80,000 difference per tonne and potentially several hundred tonnes per season. So quite strong incentives.
Looking at other food production areas, you can see similar development with plants and animals and fish would be no different.
And something to consider is that mixing of sperm and eggs which easily fertilise indicates there is quite a high degree of similarity in place, even if those fish don't look similar. Would it change people's opinions if the lima catfish and shovelnose catfish were from a single ancestor once upon a time? (Speculative only - I'm not saying they were). Or what if the lima became two or three or four different types of fish in the next 1000 years? And then someone bred those different fish with each other?