I think there are two key points in the 1920s. The 1920s were really important for this shift.

In the 1920s, when women really broke away from dresses and matchy matchy suits, and instead began to use sweater vests and other outfits, versatility entered the minds of buyers. At that point, people began to mix and match, wear more sweaters, more gored (which is a kind of skirt).

By the late 1920s, very few college men wore suits to class. The rise of the sports coat is an incredibly underlauded change in American culture. Because once boys started wearing sports coats instead of suits, men’s outfits became more versatile, they moved away from ties, they wore all sorts of different things, like sweaters, with their jackets