I know this Is an old post, but in the early 2000s 70's fashion came back in vogue -- a 30 years difference. And 30 years ago from today is the 90s, so it makes sense.
I think it's a result of the 40 year old crowd. They're a demographic with money and starting to get nostalgic for their childhood, so the market caters to that . Kids get exposed to it, a few trend setters decide it's cool/vintage, and it takes off from there.