The question whose answer I didn't see in the "Why" section is "Why 80x40?" 80x25, 80x50, and 80x43 seem obvious to me, but 80x40 feels odd... Maybe I'm just old and everyone uses 80x40 terminals now?
I'm not old, and I thought it would be a funny sized punch card. Also, his ASCII art doesn't have rounded corners like the standard IBM cards dating back to 1964.
I thought punchcards were 80x1. This is a stack of 40 cards.
Based on the link, I'm guessing "because it's roughly square in GitHub's monospace font."
Maybe because it fits inside the README area on github?