Zara, H&M, and Uniqlo are actually somewhat fashionable, at least as "value" brands. Apparently there is some effect where the top 20 remain the top 20 because they are well-known or have other competitive advantages:
https://www.mckinsey.com/industries/retail/our-insights/the-...For example https://martinroll.com/resources/articles/strategy/the-secre... lists Zara's 1-week supply chain and responsive customer service as Zara's advantages. And Uniqlo similarly has some proprietary fabric technology: https://martinroll.com/resources/articles/strategy/uniqlo-th.... Meanwhile https://martinroll.com/resources/articles/branding/forever21... lists nothing besides their knockoffs being really cheap (and the associated piracy and labor lawsuits).
I don't have time to go through all the other failing stores like American Apparel, Dressbarn, JCPenney, Gap, etc., but the story is probably similar, pointless products with no obvious differentiation. H&M is more interesting, massive chain with CEO/management trying to turn it into something more lean / innovative. It's kind of like Amazon with Whole Foods. The eventual outcome of both is uncertain but so far they're moving along.