It's an interesting phenomenon that you can observe with other things as well like tastes in music. I think it has something to do with people having access to about sixty years of people trying out just about everything they could think off. It's all been done before at this point and it's all available in new and fresh forms. So, it's no longer about collectively picking something that is genuinely new but individuals cherry-picking whatever that they like. And it helps of course that we're not funneling media through a handful of TV channels, newspapers, etc. anymore like we used to. So people cherry pick where they get their information as well.
Probably more that’s it’s much easier to market and sell something that’s been done before than come up with something new. Also our supply chains are highly specialized for the stuff we currently make. In 3-5 years we will see bell shaped pants back again as we go from 90’s repeats to 00’s repeats which were 70’s repeats. Etc etc.
Perhaps another less depressing reason is the intersection of two things:
1) For all its negatives, online culture makes it easier to find and acknowledge different groups with different tastes that share yours, you're not as subject to having to "fit in" just with those near you. Maybe another way to say it is that fewer things are "weird" because it's easy to find others doing something similar
2) The availability of styles is not quite as bottlenecked into a limited number of taste-makers like it used to be.
For me, think it's probably more your second idea, that we're not funneling media through a handful of TV channels, newspapers, etc. anymore like we used to. This creates (for better or worse) more of a "group think" mentality, since they've all seen the same tv shows and movies, and become more in sync with their views on all things, cultural, political... So people's sense of fashion is also in sync as well as the need to fit in.
back around 2002 I had some T shirts from the Gap (so, pretty mainstream) that had the visible stitching and tags look like the shirt was inside out; sort of a duplicate in that if you actually wore them inside out they still looked inside out. the only way to really tell was the little washing instructions tag on the inside.
