"Becoming the default" implies a time at which it was not the default, and I am not aware of any situation in which this is the case. Human writing is essentially always rectilinear, written in straight, parallel lines. Whether it's cuneiform, Latin, Mayan, or ancient Chinese, we've always liked our writing in orderly lines. If I want to fit the most text on a single sheet of paper, in a regular, consistent manner, then a rectangle is really the only choice. This is a logistical thing rather than one of cultural or historical significance. A rectangle is simply the maximum amount of space with any given set of two dimensional limits. If you had any reasonable alternatives, I'd definitely be interested.