I think they're asking why there aren't any Zorua/Zoroark in Sinnoh today, including this alternate form.
The same could (and should) be asked of any Hisuian form.
Extinction.
Wyrdeer needs specific environment conditions to evolve, with urbanization and climate change it can no longer do so.
Basculegion comes after absorbing the souls of fallen Basculin, if all Basculin died out or were fished completely from the wild there could no longer be any Basculegion. I also assume they have the power to evolve here too due to the presence of Giratina's power within the region.
Braviary says it goes back and forth from Hissui to winter from a more northern region, most likely the Russian equivalent of the Pokemon world due to it being based off of the Stellar's Sea Eagle which does the exact same thing in real life. Hissuian Braviary either ended up adapting completely to live in the Russian region or they simply died out due to harsh environments. Either way, they stopped coming back to Hissui/Sinnoh
Growlithe dex stats it doesn't like people which is usually opposite to its protective and loyal nature of its original form. They most likely were befriended, domesticated and bred with Kantonian Growlithe and the Hissuian form was just lost.
Kleavor evolved from Scyther due to specifc mineral deposits it needed, with the heavy mining presence in Sinnoh it is most likely this specifc mineral was over-mined and is now very rare or gone from the region.
Zoroark I honestly feel like they just died out. They were pushed to live in the harshest place in Sinnoh and most likely unable to adapt further do to their spite. As well as not looking like the most well adapted variant to begin with.
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Speaking of which, it brings up the question whether or not you can transfer in regional forms if the Pokemon can be found in this game originally. We do see Galarian Weezing in design as a smokestack in the same way Fletchling was a weathervane and they still made it into the game.
Be fun to have a Perrserker with you.