It’s because of the tools used to write a language. Some languages were written with a brush so go top to bottom. Some were written with a chisel so they go right to left.

Approximately 90% of people are right-handed. In European writing systems that use quills and pens, reading and writing left-to-right makes more sense so that you can hold the pen in your right hand and drag it rightward, not into the ink you just laid down.

In East Asia, before writing on paper was a thing, they wrote using inscribed bone, but then eventually moved to vertical wood boards, bound together by string. Each character on the board would be ready from top-to-bottom, and then move to the next board. The most logical choice for a right handed person is to stack the wood pile on their left, and use their right hand to draw the next board to meet their gaze, then set it down on their right. Later, this bundle of wood boards would become paper scrolls, but would still be pulled from left-to-right by a right-handed scholar.

For this reason, the historical writing system common to China, Japan, Korea, and Vietnam for centuries was read right-to-left (because instead of scrolls, we have pages, which can be moved easily). But the native Korean script is left-to-right, as is the modern Vietnamese script. And Chinese and Japanese in the 20th Century switched to left-to-right. And yet, Japanese books are still ordered “backwards” so that the title page is what Westerners would say is the back of the book, and manga panels are read from the right side toward the left.

So far as I’m aware, this means some Japanese signs can be rendered left-to-right (modern), right-to-left (historical standard), and top-to-bottom (traditional). The only orientation that’s disallowed is bottom-to-top (although vertical news tickers will do this, so that readers see the text from top-to-bottom).

It all boils down to right handedness, but it depends on whether your hand is moving, or the text is moving.

sigh I thought this was a riddle for a good while before I read the past the title.

It’s arbitrary. Hebrew and Arabic write right to left, for example, and other things go the other way too such as computer UIs

Additional to other answers, back in the early days of alphabetic writing, some writings alternated left to right then back again right to left on alternating lines. This is called “boustrophedon”, literally “(as the) ox walk(s)” because it’s the same way oxen are used to plough fields.

There’s documented evidence of both early Latin and Greek being written this way. What’s less clear is which direction they chose to start those writings.

The problem with that is that you have to learn to read both directions. They often wrote the letters backwards when text went the other way, which came with its own set of problems. You probably don’t have a mirror. You basically have to learn to write almost twice as many symbols. Some letters are their own reflection and you can’t always tell which way something was written. etc. etc.

Eventually someone influential will have chosen the direction for presumably a good reason (to them) and everyone else eventually followed suit.

It’s fairly arbitrary. Many languages write right to left, and some even write up and down. This will affect things like the direction you read books

AFAIK Just the way it turned out (and then was applied to more and more things). Others write top to bottom or even left to right on odd lines and right to left on even ones (boustrophedon, “as the ox plows”).