Their omnivores, (although some insist they are vegetarian, which is impossible) who choose to exclude certain meats from their diet, or they are pescetarians, a sub-group of omnivore, who may choose to eat fish because they, like my friends, see fish as "swimming vegetables", it's ludacris, really, how are fish and different from any other animal or "meat", it's still flesh, and they still feel it, and it's still mass produce, still - unnecessary.
IMO.
That's why they don't, but sometimes its because they happen to fall into that category because they only like the taste of fish flesh.