In truth, longer lines would probably be fine, except that 99% of the time when your line is more than 80 characters long, there's a better way to do it with shorter lines. I've made a habit of reducing lines to 80 characters or less, and I would guess that for every 50 times I take a line I wrote that's 80-100 characters long and figure out how to split it into shorter lines, 49 of those times, the shorter version is better. That seems like a good reason to prefer the shorter line limit.

Note that it isn't a hard limit. The actual rule is "[s]tatements longer than 80 columns will be broken into sensible chunks, unless exceeding 80 columns significantly increases readability and does not hide information." So longer than 80 is fine, but only if it significantly increases readability. That sounds about right to me!