Hadn't fully registered till this comment, the degree to which the modern web is anchored to horizontal (usually left-to-right) writing and the design patterns of vertical scrolling that come with that assumption.
It's not just the web, it's all of computing, even down to the hardware design. A vertical scroll wheel is standard on all mice. A horizontal scrolling method of some sort is not, and even on mice that include one, it's usually not as good as the vertical one (e.g. leaning the wheel left and right).