Actually,people began to wear jeans to the theatre in '73. This was when the hippie influence drove some people to dress inappropriately and it's been downhill ever since,with the lowest common denominator supplanting what had always been an upwardly striving momentum. In the Middle Ages and again during the Renaissance, kings and/or governments passed sumptuary laws, forbidding all but the highest segments of society from wearing certain materials and styles.And until the last few decades,people's aspirations for betterment of themselves and their position,had them emulating what was once called "their betters".
It is only in recent history (and no,Marie Antoinette's sometimes predilection for wearing a "sheperdess's dress",doesn't count !),that many people wear clothing that either is that of convicted criminals, IN JAIL,or that of the lowest of low rung workers,as an all around uniform.
Kind of ironic that at one time, if you tried to dress better than "your station" you could be thrown in jail for it. Despite that, people often did dress as "their betters" because they wanted to be seen more respectibly. Now people seem to revel and compete between themselves to dress as slovenly as possible in as many places as possible. What a shame.