There are a couple of reasons why Qwilfish is in NU while Hisuian Qwilfish is PU, and they all have to do with the November and December metagame in NU.
Qwilfish
I'm going to start off with Johto Qwilfish. Qwilfish having Flip Turn was nice, but the reason Qwilfish was used more than Hisuian Qwilfish was its typing. Qwilfish being a Water/Poison type allowed it to check the dominant Fighting-types of Hariyama, Tauros-Paldea-Blaze (November), Tauros-Paldea-Aqua (December), Flamigo (December), and Heracross, as well as physical wallbreakers such as Glastrier and Bruxish locked into a water move. Since Qwilfish could use an item that isn't Eviolite, it could use Rocky Helmet, which was nice to punish the Fighting-types in NU. Qwilfish's typing also helped to create cores such as Copperajah/Qwilfish or Gligar/Qwilfish, both being nice hazard stacking cores.
Hisuian Qwilfish
As for Qwilfish's Hisuian cousin, it just wasn't good in the previous metas. Yes, Poison/Dark does have its perks over Water/Poison, but in the context of the November and December metas, Hisuian Qwilfish's typing didn't help it much. Since Hisuian Qwilfish was a Poison/Dark type, it wouldn't check Fighting-types as well. Unlike nowadays at the time of the post, back in November, there weren't as much special Psychic wallbreakers back then. There were only Hoopa (which could still use Knock Off to cripple it), Mesprit, Indeedee (who wasn't seeing much use), and Galarian Articuno. Hisuian Qwilfish's typing was only useful for specfiic Pokemon such as Decidueye and specific Mesprit sets. Hisuian Qwilfish was also worn down easily. Hisuian Qwilfish depends on Eviolite for its good bulk, which wasn't good since Knock Off was somewhat omnipresent in previous metas. In addition to Knock Off, hazard removal wasn't the best, meaning Hisuian Qwilfish taking hazards chip in addition to losing its Eviolite easily wears it down. In addition to these negative traits, Johto Qwilfish was the better Qwilfish, making Hisuian Qwilfish hard to fit on teams. With Hisuian Qwilfish not getting used due to its problems it faced, Hisuian Qwilfish dropped to PU.
TL;DR Qwilfish remained in NU due to its typing being useful in the previous metas, checking relevant Fighting-types. Hisuian Qwilfish dropped to PU due to its typing not being useful in the previous metas, being worn down easily, and being hard to fit on teams.
