Hello, i'm speaking a foreign language and am here to bringing you a leak!
"DEXIT" as it has come to be known, was merely to accomodate Future Games in the Pokemon Legends series.
Pokemon Legends Arceus features countless new animations for every Pokemon included, which drastically reduce the number which realistically could be included. Hoewver, Game Freak still considers Pokemon Legends a separate series from the main series games, but the Pokemon Company International has decided they are both Pokemon RPGs and must be considered "main series." This is why there are new regional forms in PLA, early in development these were not new forms (Wyrdeer was originally Stantler ect)
Since they are main-series games, Game Freak wanted to ensure that players would not be enraged when we could not include all Pokemon in Legends: Arceus.
Galar was missing many Pokemon due to difficulty in porting the models to the new system. The Expansion Pass was to test what happened when new Pokemon were added in the middle of a generation.
The main-series "Generation Nine" games are planned to have the entire National Dex programed in. It is not yet decided if all Pokemon will be included at launch or not, they are considering making an Expansion Pass for Generation Nine.
Pokemon Legends games will have a smaller roster