Because to a user that barely understands tabs as it is, it's simply not going to fly. And yes, many nontechnical users do struggle with the idea of tabs.

Chrome on Windows has a flag that you can set to get vertical tabs, but it's a flat list (no tree style).

Tree style tabs are the #1 reason I use Firefox (though many of Chrome's other advancements are mighty tempting).

It's not going to happen because that would involve cutting a good chunk away from the side of the content area, standing in the way of pages' Manifest Destiny of extending from the left side of the screen to the right.

Having pages like that is a shitty way to browse; people who do it are doing themselves a disservice by reading lines at that width, and screens are wider than ever, but people love full screen windows.

Yeah, that's the only reason why I'm still using FF. I'd switch to Chrome months ago, but I can't stand horizontal tabs anymore.