I think part of it is the country in general has relaxed regarding judging people based on outwards appearance. I think “American” culture has generally been more relaxed than other cultures for many years. My in-laws, who lived most of their lives in Mexico City, when they meet someone new or deal with someone at a business, they almost always comment to me that the guy was dressed well, shirt ironed, clean cut, etc, like that means they are good and honest and just overall ok. I want to roll my eyes every time they say such things, but I control myself.
Cities tend to be more formal, and colder climates tend to be more formal, just for some generalizations. We have more and more suburban areas.
The computer companies and .coms were part of the initial trend on casual at work from how I remember it. Young adults created incredible technology and companies, and since they were young and in charge, they wanted to wear jeans. Then, as part of competition among employers, some other employers started doing the same. Employees in other companies were jealous of being able to go to work in comfortable clothes that could be washed and not dry cleaned. It started becoming a perk of employment like getting healthcare or vacation days.
Some jobs pay less now, so it’s difficult to ask employees to wear suits ever day. Bloomingdale’s and Macy’s used to be suit jackets for men and women had to wear pantyhose with skirts, and suit jackets with slacks. Now, Bloomingdale’s employees just have to wear black. It’s way better in my opinion, way more practical, and you can spend not very much money if you want. Also, many sales and stock positions in Bloomingdale’s make less money now then when I first worked there in the early 90’s. It started changing while I was employed with them.
One thing I’ll mention was that as dress codes started to relax in offices some of the talk was it hurt women. That men in casual clothes still had presence and were respected, but women in casual clothes were more likely to be overlooked. Also, women’s casual fashion is thought to be more tricky, more varied, more options, more ways to screw it up.
Maybe now that women are more and more equal in the workplace (I know we still have work to do on that front) maybe the power suit with heals to get us closer to a man’s height isn’t as necessary.