With this in particular I wonder if it just comes down to not wanting to just call it "Kitakami Form"
The whole situation with Kitakami continues to be weird. It's part of Paldea (it has to, for Terastalization) but it also is its own Land (there's non-Paldean Wooper, they keep saying "Land of Kitakami") but also it is NOT a region dont confuse it.
I'm gonna be frank, there's really nothing about the Terastal phenomenon that dictates it can only be used in Paldea. It's not like Dynamax where not only was the Dynamax phenomenon only seen in Galar, it was a case where even within the region itself it was only usable in certain power spots scattered across the region.
Kitakami and Blueberry Academy are both noted to be entirely separate landmasses away from Paldea. The Blueberry Academy is noted to be a place where you're "studying abroad", aka overseas in a foreign place, thus denoting the BB Academy to be foreign. and Kitakami is also foreign to Paldea. Neither are a totally new region or a new generation, but are both completely separate lands from Paldea.
Terastal was believed to have been found in Paldea and only seen there but there's nothing stopping Game Freak from saying "nah that's a lie, it's elsewhere with its own lore", and since Ogerpon is tied to Terastal as much as Terapagos seemingly, that would explain why a distant land like Kitakami has the Terastal phenomenon.
Case in point is Mega Evolution, which was originally stated in X and Y to have only been found in Kalos, but then ORAS came into the picture and proceeded to go "hah nope, Hoenn has Mega Evolution energy and Mega Stones too!" with its own separate lore about how it got there, different from how it got into Kalos.
Really I think it's just a matter of Game Freak having a shift in philosophy and these supposed "convergent" species, ie two unrelated species who became similar, being what they want to do now. Regional forms have definitely taken a backseat this gen with only Wooper and Tauros being such, while they have given more focus to convergents (Wiglett and Toedscool for instance), Paradox Pokemon, and a return of traditional new evolutions such as Dudunsparce, Farigiraf, Annihilape, Kingambit, and now Dipplin and Archaludon. Regional forms are something they are shifting away from, and they've felt they've done everything they wanted to for now in Alola, Galar, and Hisui, and are now trying out new concepts in terms of designs.
