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.