In the 1800s buttons were very expensive and it would be considered showing off to have buttons on sleeves, where they are not needed. Even those who could afford it didnt flaunt this, shelo l’haashiro es boniv. The rabbonim of the shtetls wrote a joint letter to the mechaber asking him to add this to shulcha aruch, which he did in YD siman 404. But very good question.
BTW, now you know the source for “404 error”