I was told that, way back, only the rich could afford meat. It was considered a luxury, because only the wealthy had land on which to raise livestock. Fish, however, was considered food for the poor because rivers did not belong to anyone, so anyone could catch fish to eat. Therefore, when the discipline of abstaining developed, fish was allowed because it was considered humble food, not a luxury. Of course, nowadays, seafood is considered rather luxurious and beef and other meats quite common. But the tradition still remains. We should, however, strive to avoid overindulging during Lent, whether it be fish or meat, but that is why fish is allowed on Fridays and meat is not.
If I’m wrong, take it up with Sister Angelita, my 7th grade religion teacher waaaaaay back in 1979!