The driver was that unless you have a large number of tabs, vertical tabs waste more space than horizontal tabs, due to the width of the tabs column for vertical tabs vs. the height of the tabs row for horizontal tabs. Like in this [0] random example with just single tabs, there is a lot more wasted space on the left and right (below “My Notebook” and “Phonetics”) than on the top (to the right of “New Section 1”). If we used a vertical writing system instead of a horizontal one, we’d have had vertical tabs from the start.

Widescreen monitors afford that wasting of space better.

[0] http://www.onenotegem.com/uploads/allimg/191124/12310QH9-3.g...