The 90s are just the latest era old enough to have people nostalgic for it. This has resulted in a massive boom in the popularity of the 90s aesthetic (clothes/TV shows/ movies etc). The pandemic also played a role in this boom with people having spare time and income from not going out during lockdowns etc it was filled with these nostalgic type purchases. You even had old Pokémon cards and WWF figures going for hundreds if not thousands, if still new in box, so it's a general trend that old football kits are apart of.
Football kits generally have been gaining popularity again for the past couple years, we've seen fashion forward kits dominate in recent years think the PSG Jordan kits or Nigeria stuff. As kits become more about what you can wear 7 days a week not just on a match day. Also in a era when teams release 4/5 kits a season they are turned over with such a fast rate that for many people they become a blur. Therefore for some these retro kits are ones they connect with most and thus sites like Classic Football Shirts have went from strength to strength.
You could also throw in the idea that in a society that is dominated by mass produced items, for some, vintage/retro have simply became a fashion statement for people to stand out. Limited releases of clothing are also all the rage, shoes or clothing getting sold in limited quantities get flipped for 2/3 times the retail price have created this also fever-like market of you have to have it at any cost. And I suppose the ultimate limited supply is stuff they made 20/30 years ago. So I suppose there is almost like a statement of saying of yes I managed to find this 25 year old shirt, you know I probably paid a pretty penny for it but I'm nonchalantly going to wear it.
Then you add the general Rangers dynamic to this situation with the retail boycott and the hell that was 2012 to 2020. So many turned to kits from happier times and that in itself created its own demand.
The Dhgate/fake/unofficial remakes bring out an interesting dynamic to this whole debate. From the Rangers ones I've seen they are very easy to distinguish when comparing to the real deal. But that's probably because they are trying to remake kits from 20+ years ago and don't care if the sponsor or design isn't 100% accurate and for £15 some will put up with it when the real ones go for £80 plus. But I'm the kind of fanny who would know to check these sort of small details out and they can be dangerous to the average Joe if searching on eBay thinking they've got a deal when they've spent £50 on a fake. I've even seen people say that they've became too common that they don't wear the real one now because people will think it's fake at first etc.
Overall, nostalgia sells and football kits are in vogue