Probably because with something like prawns, you get ~15g protein per 100g whereas beef you get ~25-30g per 100g (depending which site you look at) so you'd need to eat twice as much prawns as you would protein to get the same amount of protein.Not only that but beef has many other nutrients along with being fatty at the same time so you don't need to add an external fat source.
This pretty much hits the nail on the head.I do like some shrimp cooked in butter and garlic sometimes.