I think it's cus they're more likely to get dressed up before going out, since fashion is a lot more valued in big cities than it is in smaller towns. like the idea of "dress to impress" is a lot stronger in bigger cities since you never know when new opportunities might pop up.
in a smaller town? I don't care how I look when I'm going to the grocery store and "fun things" are further away, which means a long drive, which means I'm wearing comfy clothes and I don't care.
I also know more people here, so I'm not looking to impress them. I'm gonna see the same people at the store as I do passing by when I'm doing yardwork, so it doesn't matter that much.
it's like first date clothes versus 100th date clothes. on the first date, it's a stranger, so you wanna impress them. on the 100th date? you know each other pretty well (hopefully) and you won't care about wearing sweatpants.
same with a big city, it's like an endless first date so you wanna look good.