in the original 13 colonies wealth was fairly concentrated in wealthy families that had moved from europe. these continued to be the upper class for most of american history and their wealth expanded with industrialization and colonialism after american independence. this cultural class you can call Yankees. over time some new money joined the old money, guys like carnegie. but for the most part new money had to sort of adapt to the old money ways and overall the east coast industrialists turned out just like the older yankees in dress and culture. in a lot of ways yanks were just europeans.

much of the south never economically recovered from the civil war and it's upper class, that somewhat resembled the yanks prewar, began to decay. the real wealth creation in the south didn't happen until oil and natural gas started being drilled in texas. suddenly the wealthy there were guys who just owned lots of farm land, once-settlers who had moved west and didn't necessarily ever posses any wealth, and certainly not the kind of prep school wealth the yanks had. newly minted millionaires were never exposed to prep school or matching leather or any of the preppy menswear stuff you still hear about now.

these new rich men from texas had the wealth to impact culture and politics and preserving and increasing their wealth was a driving force in their politics. there were maximum quotas for oil extraction and the party unified around raising them. basically with southern money entering politics the yankees were finally challenged and with this power shift came an end to yankee dominance in culture.

the yank aesthetic was oppulent but less showy. yanks hung out with other yanks and all did yank shit together. texans got rich and were still surrounded by poor texans and the oppulence was a bit less... subtle. precursor to the mcmansions and bullshit of today, stuff that's uniquely american.

tl;dr basically blame texan oil money for the walmart fashion of today.