(Hopefully not showing my age too much, but) I originally heard that n years ago as "more than a single page" when printed, but the sentiment survives. If a sub is spread over more than a single "contextual unit" (printed page, screen full) you start to spend more time trying to track where you are in the big picture rather than following the flow of the logic.

(Of course when you run a 1280x1024 monitor rotated to portrait orientation "a single screen" can get pretty long . . . :)