Hey.. I still remember my first programming experience with Turbo Pascal running with MS-DOS 5 and I didn't know how this 80 characters per line popped up in history. I remember there was something like .. "MODE CON..." that I could change the chars, but I didn't know why and how the defaults were settled everywhere the same. Kudos for the OP asking this question.
The train of reasoning here reminds me of another story (which I think has been on HN before): Why is a space shuttle engine 4' 8.5" wide?
http://www.astrodigital.org/space/stshorse.html