Is there a reason or is it arbitrary/tradition?
It was shown, by experiment, that optimal amount of text for print material is ~60 characters per column. This was calculated by the ratio between font size and leading which were picked by humans as most comfortable to process. Higher amount of characters per column interferes with brain's ability to scan through text easily, much like you need to break text into paragraphs for same reason.
There's long-standing rules of thumb, and now quite good psychological research, to indicate that ease of reading requires limited line length. The rule of thumb is somewhere in the neighborhood to 43 to 60 characters per line, or around ten words per line in English.
Newspaper and magazine print is usually around 10 pitch, ie, 10 characters average per inch. Printable space on a page is usually around 6 inches wide in a letter-sixed page, 4.5 inches in a digest, 10-12 inches wide in a newspaper (tabloid vs the other format whatever the hell it's called) and the number columns of print is somewhere around (inches × pitch)/50.
(Speaking of references, the wiki page is actually quite good.)
My guess would be that it is to counterbalance a cost-saving measure. They needed to have the words be below a certain size so they could fit more of them per page; and in that quantity a single column would just look like a big chunk of text. So the reasons are two-fold; one, they reduce the size of the words to save costs by using fewer pages, and two, they partially offset the visual problems associated with having text that small by breaking it up in a way that's easier on the eyes.
The only source available for the 60 characters per line experiment that I could find appears to be from Tomás García Ferrari & Carolina Short done in 2002.
See Test 3 in this document.
I think when Keyframe says "optimal amount of text for print material" that this is related to scanning and absorbing key words, but not deeper reading comprehension levels.
I know that when I'm reading a novel or a even a non-fiction book I pay closer attention than when I'm reading the newspaper. It might be that ~60 characters per line is optimal for that level of casual reading.
Earlier comments characterize 60 characters as being comfortable. I've believed it was to specifically increase reading ease and speed by enabling one to slurp in an entire line. For me, that is comfortable.
I tend to shrink the width of HTML windows that have reflowing text, so that I can gobble single lines. I find the width of an eBook line on a mobile device to be good.