Going back in time it's likely because the Sunday paper was traditionally the largest edition of the week. It would normally have a lot more sections to it. You'd have special features like the Sunday comics and they would be in color, not black & white like the rest of the week. The paper would usually be a bit more expensive as well ie. $2 instead of $1. I'm guessing it likely reached a larger audience.

lol, I feel old now talking about this :)

This made me look up when the rules about posting in the newspaper went into effect. I was surprised to find that it was 2005. This is much later than I thought. I would have guessed the 90s. By 2005, it was clear that the newspaper industry was dying due to the internet. I guess this is one example of how out of touch government can be.

Someone else mentioned in the thread elsewhere about whether the rules would need Congressional approval. Gemini says that these are purely an agency regulation and the Department of Labor can update them at anytime. It's kind of mind-boggling that no administration has updated them in 20+ years.