Did you ever wonder why magazines and newspapers use multiple columns? Because it's easier to read. When lines get too wide, it becomes harder to scan from the end of one line to the beginning of the next. There's solid science behind that, based on both physiology and observation of effects. For text, the recommendation is generally as low as 50 characters or as high as 75. Yes, reading code is a bit different, but not so much different that the same principles don't apply.

80 wasn't chosen, or kept so long, arbitrarily. To overcome the issues with both readability and ability to support side-by-side diffs or multiple files on smaller monitors or for those with poorer eyesight, there'd have to be a supportable positive reason to go higher. So far all I've seen is personal preference and occasionally support of bad habits (e.g. "trying to do too much in a single line" as another commenter put it). I just don't find that very convincing.