While probably not the original reason for the 80 character limit, a reason that it was accepted widely is simply reading ergonomics:
- If lines are too short, text becomes hard to read because you must constantly jump from one line to the next while reading.
- If lines are too long, the line jumping becomes too hard because you "lose the line" while going back to the start of the next line (this can be mitigated by having a bigger inter-line spacing, but this also wastes space).
This is widely known and accepted in typography. The standard recommendation (for text in books etc.) is to use something in the region of 40-90 characters per line, and ideally about 60 (see e.g. Wikipedia, Markus Itkonen: Typography and readability ).
If you aim for 60 characters per line, your upper limit must obviously be a bit higher to accommodate the occasional long expression (and things like margins markers and line numbers), so having an upper limit of 70-80 makes sense.
This probably explains why the 80 character limit was taken over by many other systems.