[pocketarc]

I believe the reason is Catholicism. A sibling poster mentioned it being a thing in Poland as well (catholic), and I remember when I was a kid, on Fridays you would not eat meat, but fish was OK.

Having that tradition, that every Friday you don't eat meat, but fish is acceptable, seems to colour your perspective when thinking about what is being a vegetarian.

[JetSpiegel]

That's before Easter only, it is supposed to be about eating plainly. The 'no meat' thing is an historical residue, some fishes are actually more expensive than chicken, for example.