It's fairly traditional. Monday and Tuesday were typically quieter, so if you wanted to take a day off then you'll do it when it'll have the lowest impact on your livelihood.
Then when chain restaurants arrived and opened every day, prioritising availability of their product over making every day profitable, that pushed more of the smaller ones into offering the same service.
So everyone is right. You don't seen it very often, but that doesn't mean that it isn't a thing.