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Why do most of today's websites, especially news and blogs, use mostly vertical scrolling?
Also why has it become a pattern, that the more you scroll down you find older material, rather than scrolling from left to right (which in most of the world is also the natural reading direction habit and the natural way to draw a timeline from past to the future)?
Do you think an interface like TheWall would work by scrolling to the right instead of the bottom?
Horizontal scrolling like all other features requires certain conditions to exist in order to be comfortable. These conditions include display technology (screen & layout) and the navigation control technology (input devices).
Let's look at the brief history of them all.
Nearly all analog means of presenting information have had horizontal scrolling: be it old-school parchment scrolls or bound books ("pages" were turned along the height not width). (I think there also were vertical scrolls but I couldn't find any photos of them.)
When we jump to the digital age, we find that the video chips of early computers had rather tiny resolutions (e.g., Commodore 64 had just 320 × 200 pixels, which translated into barely 10 lines per screen). The second peculiarity of early computers (I've skipped the punch-card age) was the lack of a GUI, which meant that input commands and output where all typed and presented into the same screen.
I guess the thinking of the original CLI designers was along these lines:
When we reach the end of the line during input, we push the entered text up and continue typing on the new line. When the screen fills up with text, let's just keep adding more lines as we need. This way we'll have an infinitely tall sheet of paper that we just need to scroll up or down somehow. Awesome!
Interestingly, the aforementioned Commodore 64 didn't even have dedicated page up/page down keys (according to some manuals, they were coupled with the F-keys on the right).
Jumping another 10-15 yeas into the future to late 1980s and the first GUIs, we see computers conquering their place in everyday office life with spreadsheets and rich text editors, the latter being the modern reason for vertical scrolling. The low resolutions (even with IBM's high-end 640 × 480 VGA in 1987) couldn't display one letter-sized page (or A4 in Europe) per full screen. There was no other way but to scroll vertically to read one page. To remedy the situation, Microsoft invented their zoom/scroll wheel in 1993-1994 and Apple patented Mighty Mouse with thumb scrolling in 1992. The mouse controls were obviously made to follow the pre-existing scrolling paradigms.
Even today, vertical resolution isn't high enough to fit a letter page despite allowing 2 pages horizontally. This is a screenshot of Microsoft Word 2007 on a 1920 × 1080 (save for the Windows 7 taskbar):
The Internet as we know it was born in early 1990s and had to follow the conventions & paradigms of desktop applications, which weren't easy to defy when the keyboard had Page Up/Page Down keys. The conventions influenced not only design guidelines but also the architecture of HTML rendering: DOM elements are placed to the right of the previous element as long as there's unused space and then shifted to the next line below it increasing page height. Yet, even vertical scrolling on the Web wasn't recommended until 1997.
In the recent years a few important developments have happened, which signal a potential shift toward horizontal scrolling in the future:
However, horizontal scrolling is still far from adoption on full-sized screens. A study presented at WWW2009 (PDF) found that users preferred vertical scrolling because they were used to it as opposed to horizontal scrolling and some pointed out that horizontal scrolling because it "forced them to move their eyes vertically up and down the full height of the screen", which isn't the case with vertical scrolling. The latter reason is a strong indicator of ergonomic preference for vertical scrolling.
Horizontal scrolling is terrible. Users almost always complain about it, and it's particularly a problem when paired with vertical scrolling; there's nothing worse than trying to scroll a site on mobile when you keep scrolling horizontally on accident.
Lots of data backs up this idea in web user's behavior; Horizontal Attention Leans Left (for Left to Right languages, of course). By keeping everything upright, vertical scrolling allows for scanning and a clear hierarchy that's not always possible in a horizontal layout.
So no, I really don't think horizontal scrolling works as well if only because of the conventions and expectations. Plus, our input methods are now adapted to vertical scrolling more than horizontal; vertical scrollbars on mice, page up/down keys.
As for why it became a pattern, I think many things are factors.
You bring up reading order...we do read left to right (in English ect), but there's an optimal line length; you don't just stretch all text onto a single horizontal line, even if you could. So at some point you end up with a vertical column. You can split text into multiple columns horizontally as is common with books, but that's necessary due to the limits of paper. Horizontal pagination doesn't make any more sense on a computer by default than vertical.
The old days of Command Line Interfaces is probably where vertical scroll started; new lines always worked downward. It's easiest to keep scrolling to one dimension, and typwriters started a convention where you're always scrolling down until the end of a page. Of course, on the computer the "page" can be of arbitrary or infinite length. People knew how typewriters work, so it made sense to make Command Line Interfaces work like typewriters in many ways. When it came time for the GUI, it made sense to keep some conventions of the CLI; IMO vertical scrolling is one of the few applicable conventions to carry over.
Horizontal scrolling generally only works when the problem-space requires it (maps) or you're following a paginated format; emulating pages can work well, but there's often no need to in a digital format. Some conventions in old media don't apply and were created out of necessity, not because they make universal sense.
On clear advantage to vertical scrolling is that the user gets to control which portion of the text is on the screen.
If I'm reading a book, (or a document set up with pages that I need to scroll horizontally through), then I often find situations where I am flipping back and forth between pages to try to mentally connect two portions of related text (or text and illustrations). With vertical scrolling, I can simply adjust the page until both parts are visible.
The user controls what parts are important to see simultaneously, not the page boundaries.
We read lines of text left-to-right but we read from top to bottom. Though not exclusively throughout history, for the most part, most documents have always been vertical in nature and one reads them from top to bottom.
In western languages, the common reading direction is left-to-right, and then top-to-bottom. While I don't have any sources handy, pages of text, long long ago, were set up this way because the human field of vision and ability to scan is much more suited to looking left-to-right much more than looking up-and-down, making "lines" go left to right.
That is where the basis of text layout in left-to-right, top-to-bottom format comes from, and then when the transition was made to digital, print artifacts were, in general, just copied wholesale, since digital was just an extension of print initially.
Given human vision mechanics, coupled with western language prevalence in the computing world, it's no surprise that the same format has "stuck" when large amounts of linear data need to be quickly scanned and processed.
I think it is interesting to note that we may soon see a break from this pattern of vertical scrolling. Windows 8 seems to focus on horizontal scrolling for content. That has already started in Windows Phone 7, but judging by the consumer preview of Windows 8, we will see a much stronger focus on that in windows 8. Content is presented in columns, that can be scrolled towards the left to reveil more content on the right.
In my trials with the system on a normal laptop, I did not find it particulary plesant, but I think that is mainly due to the limits of using a non-touch optimized system. I think that on a tablet with touch enabled, it may actually work very well. Still, I'd need to try it, and even that is very far from doing actual proper user testing.
Vertical scrolling pattern may come from a newspaper legacy. Indeed, wrapping text into thin columns let trained readers to catch a complete line (thus 4 to 7 words) at only one glance.
Moreover it seems exhausting to zigzag from left to right with your head to follow a text, than just going from top to bottom of a document.
Now that we are all used to this pattern, I think that would look very strange to change it to horizontal, unless each piece of text/article is wrapped in a column, and all column are layed out horizontally. So following this idea, I presume a horizontal TheWall would be surprising but still easy to read.
This answer augments those already given by Ben Brocka and AShelly.
Connect the convenience of accessing any given portion of the text (mentioned in AShelly's answer) with the optimal line length (mentioned in Brocka's answer) and you have vertical scrolling as the only sensible option available.
You cannot have unending lines. Imagine reading a book on a tablet (and I say a tablet because it's one of the platforms where scrolling either ways is just as easy*) written in only one line. You can scroll horizontally all you want but you can't escape the misery of it.
Another thought experiment I would like to suggest is to imagine reading this Q&A, with the answers and the subsequent comments, lined up horizontally. It doesn't make any sense.
*Please spare us the biomechanics lecture in case you are tempted to say that the horizontal swipe is easier (or the other way round). I mean "almost" just as easy.
When I am asked why we support vertical scrolls vs horizontal scrolls, I answer this was a convention which was followed as a result of how screens looked at the time and even more importantly how information was generally presented. Trust me there were sites even in 2000 or earlier than that, that offered horizontal scroll too, but the convention was to follow the successful or most prevalent, the mouse-wheel's behavior further extended this practice.
Today with wider and even wider monitors, we have the choice to change this paradigm, and a lot of experimental websites are going that way, if you notice win-8 also has a horizontal scrolling paradigm employed. Both top-down and left-right are normal convention's user's follow so both of them are successful.
