To paint a picture of fashion lore...
F21 was the trailblazer of online and in-person retail fast fashion ~15 years ago. If you didn't know yet if you were an American Eagle, Abercrombie or PacSun kind of girl - and you had not yet "aged into" the Zara or ASOS paradigm, you could do a one-stop shop at F21 and get a taste test of high quality garments in various styles at a decent price point (and they offered lucrative sales). However, F21 garment quality dropped significantly about 10 years ago and F21 became increasingly associated with the devil's "fast fashion."1
So as this fabled consumer grew up, alongside the Zara/ASOS paradigm, the drumbeat of "ethical fashion," came to the rescue. The market share was originally dominated by American Apparel (that's a story) and eventually evolved into even higher-end alternatives such as LA Apparel, Everlane and Reformation and up-priced ASOS and Zara options. Standard "grown-up" labels such as Anthropologie also play into this marketing quite a bit.
For the consumer aging into the F21 demographic today, boutique options such as Brandy Melville are more attractive as far as more high-end in-store shopping (with higher garment quality - F21 is essentially a dirty word at this point), and Shein and Fashion Nova seem to dominate the online marketplace for more "flirty" or "daring" styles. However, Target's younger brands such as Wild Fang and A New Day actually capture a lot of the current F21 demo interest with on-average higher garment quality.
Today's online consumer fashion2 discussion communities are increasingly preoccupied with "ethical fashion," even for younger consumers. This generally splits into two factions, the company or brand route (even Target is preoccupied with selling its garments as "less wasteful") or the 2nd (3rd and 4th-hand) marketplace and the growing "up-cycle" marketplace. Interestingly, consumers geographically distributed seem to concur that Goodwill and Savers prices are extraordinarily high, perhaps reflecting demand.
If I had to name it, F21's garment quality dropped dramatically, they never made a successful jump into cosmetics3 and Target captured a good majority of the scraps.

Footnotes

  1. The dirty word "fast fashion" entered the vocabulary about 10 years ago when it began and continues to warp apparel marketing.
  2. "Real" high fashion commentary - in the far-off culture magazines in the clouds - hilariously seems to still come down to textile innovation.
  3. People seem to be buying more cosmetics than clothes - when you look at what's purchased new (you can't buy cosmetics used, ew) - even Macy's seems to be transitioning to more cosmetics as the department store decays.