The practice never made any sense to me. Fish is a meat too. I think the Catholics should have gone with eggplant.
History and sociology lessons, Demonic.
Everything that exists, exists for a reason, even if one doesn't understand the reason.
The thing about Roman Catholics having to eat fish on Fridays dates from just before the medieval era, and it came into being pretty much the same way the more-ancient Judiac tradition of scorning swine evolved.
Customs are not created by a bunch of old men sitting around a fire decreeing this or dictating that.
Customs spring from the collective subconscious, or from God instructing us on our best means of survival.
Either one.
Just as the ancient Jews decided it was probably a bad idea to dine on swine, without consciously understanding why (the eco-system of the Middle East, and how swine would have ravaged it if the Jews had developed a taste, and hence a market, for swine), the medieval Roman Catholics decided it probably was a pretty good idea to dine on fish occasionally, without consciously understanding why.
These collective subconscious ideas had to be made into "laws," and enforced.
There is no one here old enough to remember "goiters," but goiters at one time were an enormous and common hazard to health, causing all sorts of pretty bad ailments and premature deaths. Goiters are an easy thing to avoid; a trace amount of iodine once in a while, and no goiters.
(Since the 1920s, Morton's table salt has included iodine, so the matter's moot any more.)
When I was younger, I knew an old retired country doctor (mostly Kansas), who had practiced during the 1940s, 1950s, and 1960s. He had treated tens of thousands of patients, Kansas rustics. All the patients he recalled having, that had goiters were.....Protestants. He could not recall ever treating a Catholic for goiters.
There's a lot of customs that seem ridiculous to some--the ancient Egyptians making cats into gods or deities, the sacred cows of India--but these weren't customs created out of whim and caprice, just for the fun of it. There was a reason those customs evolved, their end consequence being to help a particular society survive.
Call it "collective subconscious" or God instructing man in the best ways to survive; either one works.
All that exists, exists for a reasonable reason.