I don't know if this could be relevant, but some cultures have a religion that allows them to eat vegetables and fish. Some branches of Buddhism, if I remember correctly. Thus less people motivated to work on fish replacements.
if you consider ranking living things by their capacity to feel or understand pain, you end up with fish being lower than large land animals. Also humans more often can develop relationships with land animals than fish, while we can develop relationships i.e. with dolfins; we don't eat them often.
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