No matter how cool you may have thought you were in high school, when you look at your yearbook a decade or more later you are likely to laugh, not just at yourself but also at all the other cool kids. That's because clothing styles and hairstyles change with time, making earlier styles laughable. Fashions change in other kinds of things, also, such as cars, furniture and architecture. One thing we don't usually think of as going out of fashion, however, is language, and yet it does.
Read something written 200 years ago and you are likely find that the past tense of swim was swimmed, not swam. The past tense of dig was digged, not dug. The past tense of hang was hanged, not hung. Today's those older past-tense verbs seem laughable. They are long out of fashion. If we could go back in time, proper English speakers would likely laugh at the way we talk.
Pronouns, too, change with the times. Today it is fashionable to use they as a singular pronoun, when our English teachers probably taught us to use either he or she, and when in doubt, use he. That was the fashion then. But if you read William Shakespeare and certain other writers from earlier times, you will find they used they as as singular pronoun, too. This is a fashion, like hemlines, that goes one way, then the other.
Reading a Jules Verne novel recently I was reminded once again that writers of his era, including Sir Arthur Conan Doyle, were fond of the word singular. Some people still use that word on occasion, but today we much prefer unique. The use of singular has simply gone out of fashion. So have the words thee and thou.
You may have heard of Strunk and White Elements of Style or the AP Stylebook. These are guides for English usage, the latter often used by journalists, the former by writers in general. The word style in these titles is suggestive of the fashionable nature of language. The writers of these guides are saying, in effect, there may be other ways of saying this, but this is the way we approve. This is what's fashionable here and now.