Why Pokémon Legends: Arceus' Hisui Region Isn't Called Sinnoh

screenrant.com:

Pokémon Legends: Arceus is set in the region that would later become Sinnoh, but in the time the game takes place, the people of the Pokémon world called it Hisui. Legends: Arceus goes to great lengths at setting up how Sinnoh would come to be, but it spends little time explaining how Hisui — or later Sinnoh — earned its name. However, looking at the Japanese meanings of this region's names and the history of the Diamond and Pearl Clans reveals a layered reason behind why the names Hisui and Sinnoh were chosen.

In Pokémon Legends: Arceus, the Diamond and Pearl Clans represent two religious groups who have differing opinions on the god they call «almighty Sinnoh.» The Diamond Clan claims this deity Sinnoh created time, while the Pearl Clan claims it created space. Of course, the Pokémon the two groups believe in are Dialga and Palkia respectively, but they do not know this yet and thus believe the other group holds faith in a false deity.

Related: Pokémon Legends: Arceus: What Happens To Volo

This is the major reason why the Hisui region is not called Sinnoh in the game: The Diamond and Pearl Clans only recognize the god they call Sinnoh is actually two gods they can respect together by the ending of Pokémon Legends: Arceus. However, the name Hisui also comes with a lot of its own interesting subtext. Though the origins of the name Hisui are not explained as clearly as what birthed the name Sinnoh, analyzing the Japanese meanings associated with Hisui make it clear that it is the perfect name for the region.

In Japanese, the word hisui has many meanings, several of which are relevant to Pokémon Legends: Arceus. The most significant meanings are «halcyon» and «Sun.» Halcyon — which refers to an idyllic time in the distant