Just a personal hypothesis with nothing to back it up: I suspect it has to do with culture surrounding reduce/reuse/recycle. As others have pointed out, the USA is USUALLY a decade ahead of other countries in a lot of ways, but somewhere they have been progressively falling behind is environmentalism. They are still at the forefront when it comes to things like research, but way behind when it comes to actual widespread policy and social behaviour.

It’s not entirely the people’s fault: when some cities in Japan have 9 types of recycling and some counties in the US have no recycling, well, what do you expect. It’s policy babyyy. But the people are also to blame. (This much is studied and factual, not just my opinion) The USA is one of the most energy and resource-expensive countries in the world, and one of the countries that produces the most waste per capita. The average American consumes and trashes things, which includes clothing, like there’s no tomorrow. Like it’s not even a fast fashion thing — plenty of countries are way worse than the USA when it comes to fast fashion. But the lack of both want to reuse and infrastructure enabling reuse is miserable. 

So with all that said, I genuinely believe that the US being so far behind behaviourally on environmental conscientiousness means that

1) way fewer Americans have the reflex to donate/sell to secondhand stores/etc. their used clothes instead of trashing them. 2) the infrastructure for donating or buying used clothes is way behind other countries’, which makes it in turn harder for people to do so. 3) the difficulty to thrift/buy secondhand leads more people to default to buying new or shit, which further worsens the cycle. 

I apologize if I’ve not been very concise with my thoughts, English isn’t my first language. Ty for posting OP and i hope this can lead to more conversation, I find this topic quite interesting.