If it's not done a lot on the web, I blame CSS. With horizontal text you scroll the page from top to bottom, with vertical text from right to left. Most HTML/CS

If it's not done a lot on the web, I blame CSS. With horizontal text you scroll the page from top to bottom, with vertical text from right to left. Most HTML/CSS seems to be optimized for the former. E.g. vertical percentage margins don't work like horizontal ones, and CSS3 columns have similar issues.