1. Screens are usually wider than most web pages usefully support. This uses up space that would normally be wasted.

2. Most screens are wider than they're high. This is especially true of laptops. So using vertical space for a horizontal strip really eats into the vertical real estate.

3. Most written scripts are horizontal. As a result, lists are usually arranged vertically. This aligns with how lists in nearly every other context are arranged (how many times have you found a list where the second item is to the right of the first, the third to the right of the second, as opposed to them being on new vertically arranged lines?)

4. Since the text in most languages flow horizontally, it's trivially easy to adjust the width of a vertical tab container to customize how much of the text you want visible. This could range from really wide tab containers so you can see the entire title (which the larger width of monitors makes almost cost free) or you can make it really narrow to only include the icon, or somewhere in between. Arranging tabs horizontally provides no such easy and obvious UI to do such a thing, so you're reduced to either seeing fixed size tabs or icons only, controlled largely by the browser.

5. Again, because of horizontal text, tabs are shorter than they are wide. You can fit a lot more tabs in a vertical tabbar while still displaying their text than you could in a horizontal one.

6. It's the only alternative if you want to keep UI elements out of your title bar.

7. Vertical tabs can easily have nesting (subtrees), just like those file manager left hand side windows. So you can structure your tabs better.

Theoretically also possible with horizontal tabs.

But not as scalable. You either have a very wide horizontal ribbon which needs a lot of side scrolling, or a shallow depth that doesn't allow you to group as much.

There's a reason every tree/file explorer in existence is vertical :-)

I've got an ultra wide display and more horizontal than vertical space.

Also most websites scroll vertically and it feels better to have more in view at the same time. After 600px horizontally, most sites just render white space.

Because you can fit more information and vertical scrolling is easier (you also have a bigger area to scroll in), so navigation is also more convenient

Screens typically have much more horizontal space but ideal page text width has a limit so the sides end up as unused space. Also tab nesting can be very useful for organization.

Horizontal tabs become a pain with more than a handful of tabs open, particularly on small screens. Vertical handles any number of tabs gracefully regardless of screen size.

You'll find it's usefulness relative to the width/resolution of your screen and the amount of tabs you tend to have open at once.