I thought the historical reason was that punch-cards had 80 characters per line.
...just Googled it: http://programmers.stackexchange.com/questions/148677/why-is...
...just Googled it: http://programmers.stackexchange.com/questions/148677/why-is...
It's like the railway gauge size being standardized by the ancient romans. The best part of all of this though is the other comments on this page - many of them seem to imply that 80 width was chosen because of how well it fits onto your computer screen and that 100 must be too wide. If the dollar bill in 1890 had been a bit wider, they'd likely be arguing now how 80 is too small!
It proves the point that it's important to actually understand why certain things are chosen and not just assume they're always the best. Going with the standard method is not always the most beneficial path.