Because it's unreadable, and virtually every directory has more files than fit in one screen, so you either read it in an unnatural left→right up→down direction

Because it's unreadable, and virtually every directory has more files than fit in one screen, so you either read it in an unnatural left→right up→down direction or you read it in the natural up→down left→right direction and keep scrolling back up and down. It's just a terrible default that really has no use in a text environment.