I think this is due to the desire to fall in the approx 40-75 characters per line that are considered ideal (numbers vary depending on the source of the study but broadly fit this definition) for users to read. It is also why some prefer double-columns in academic papers, because each 'column' has closer to ideal lines, rather than have wide margins on the page.

Of course, this looks like wasted space when you have a landscape-oriented monitor... I personally very rarely maximize my browser so that I read in more of a portrait-style mode, regardless of the orientation of my display.