Despite the fact that I don't know of anywhere else 75 CPL is common, it's a number that tends to come up a lot.
From Robert Bringhurst's The Elements of Typographic Style:
"Anything from 45 to 75 characters is widely-regarded as a satisfactory length of line for a single-column page set in a serifed text face in a text size."
Other references cite similar numbers, the bottom end hovering around 35-50 and the top end often right arount 75 CPL.
... of course, most of these style guides have nothing to do with code on a computer screen, regardless of how many times people suggest that the column widths used by newspapers are univerally optimal for readability and have nothing to do with easing the task of page layout with the necessity for image and advertisement insertion.
In the end, I have no idea why it's 75. If it came from MATLAB's foundations in Fortran or Java, you'd expect it to be higher. Maybe 75 columns is the usable width of an old display with a post-it note obscuring the edge of the screen.
