One theory I saw argued the punch card size was the reason for 80x24. But why were punch cards that size? They were designed off of the cards used for the census. Why were the census cards that size? Because they were modeled after the dollar bill size.
I do love thought experiments like this but do believe they’re insatiably unresolvable.
In the end, all reasons resolve to either "it's what we had at the time" or "someone thought it looked good."