It is a common convention that webpages scroll up and down instead of right to left, but all printed material that's not written on rolls of toilet paper 'scrolls' horizontally. Why did the web break this ancient convention?
The main advantage seems to be the elimination of the vertical jumps that occur when we 'turn the page' as we have to move from one vertical column to the next. This is probably easier on our eyes, and it removes concerns that these jumps might occur at an inopportune place in the content (makes things more uniform). In addition, books probably scroll horizontally because it's easier to hold them open when the pages meet in a vertical line at the center rather than horizontally, so that probably outweighed any other UI concerns in the evolution of books.
Are there any other UX reasons why web pages and word processors almost always scroll predominantly vertically?