[jwells89]

I believe it’s likely due to usability issues on increasingly common small laptop screens. On a 12/13” screen for example hierarchal sidebars become a truncated mess after only 1-2 levels of nesting unless the sidebar is expanded and eating up valuable main content space.

Personally even 1-level vertical tabs are valuable because labels don’t get truncated or hidden nearly as badly as they do with traditional tabs, plus vertical scrolling is more natural and effortless than horizontal is. Additionally, most screens these days have tons of width while height is at a premium, and vertical tabs takes advantage of that.