That is an interesting question. When I was young I had a good friend who was Catholic. I spent a lot of time at her house and shared many meals with her family and if it was Friday, there was always fish on the menu but never meat. I was curious so I asked her parents to explain the custom to me. This is what they said.
When Jesus was crucified and died on the cross he took his last breath on a Friday. We now call that date Good Friday. To honor His sacrifice, the Catholic Church said that believers should make a small sacrifice of their own on Fridays by giving up meat. Why meat? Well, for one thing, meat was considered a luxury. You could only have it if you were wealthy enough to buy it in a store or raised it yourself. Anyone, however, could grow a few vegetables or catch a fish from a nearby river or lake. That way, even the poor could still have sustenance while honoring their religious beliefs.
Here’s an interesting side note. It was because of this Catholic tradition of not eating meat on Fridays that McDonald’s developed their Filet-o-Fish sandwich. One McDonald’s franchise owner in Ohio noticed that business dropped off sharply on Fridays because the Catholic customers were staying away. When he created the tasty fish sandwich offering they came back and business was booming again.