It didn't flop anon, GW canned it because of copyright.
GW has a history of bullying smaller companies in court over their IP, and it bit them in the ass twice. They tried to sue an author for a book called 'Spots The Space Marine', claiming the owned the copyright for 'space marine' (they didn't, they took the name from Starship Troopers), then tried to sue Chapterhouse for making models of units that didn't exist (court ruled you can't copyright something that can't exist).
So GW started cracking down on making sure everything in their IP was under copyright. Space marines (not copyrightable) were renamed Adeptus Astartes (copyrightable). Guardsmen (not copyrightable) were renamed Astra Militarum (copyrightable).
Fantasy however was a kitchen sink fantasy setting. Because it was inspired by classic fantasy it couldn't be copyrighted- anyone for instance could make viking-inspired dwarves. As there was no way to copyright the entire setting they instead nuked it and replaced it with AoS, which was designed by copyright lawyers.
Tomb Kings were nuked because you can't copyright ancient Egyptian aesthetics. Bretonnia was nuked because you can't copyright medieval knights. Everything with a generic name was renamed so that it can be protected by copyright- so instead of the (uncopyrightable) 'dwarven engineer', you instead have a 'duradin endrinmaster'.
Notably, the one Fantasy race that didn't suffer renaming and redesigning were the Skaven, which were an original IP and thus under copyright anyway.
So nothing to do with Fantasy's sales, entirely because GW are a bunch of greedy bastards obsessed with copyrighting their IP to an extreme degree so that they can win court cases.