Intuitively, it might be because the mouse predates the touchpad, and most mice only have vertical scroll wheels (so it conditions users to avoid discovering ho

Intuitively, it might be because the mouse predates the touchpad, and most mice only have vertical scroll wheels (so it conditions users to avoid discovering horizontal scrolling as an option, unless you click and drag a horizontal scroll bar). Currently, it's also rare for webpages to feature horizontal scrolling, so by habit most users scroll vertically to access new information. Discovery is also another reason mentioned by users elsewhere in these comments. Since horizontal scrolling is a rare behaviour, a clear signifier [0] is needed to quickly communicate that horizontal scrolling is a possibility, which is an extra problem that other potential methods don't have. [0] https://sites.google.com/site/thedesignofeverydaythings/home/signifiers https://sites.google.com/site/thedesignofeverydaythings/home...