>>50376298Here's the thing about that actually.
The Gameboy Games are effectively in their own deprecated era from when they thought Pokemon wasn't going to last past them, ergo, the series was going to be finished after GS. The narrative was that Oak finalized his 1996 Rendition of the Pokedex with 151 known species, but between that year and 1999 (the year of GS' events), 100 more species began to emerge or prove viable enough to include in the next rendition of the Pokedex.
But from RS onward, courtesy of Pokemania assuring the franchise would never end, they made retcons to the Pokemon World. Starting from RS, which had the excuse of being in a new region separate form Kanto, they've made it so most "new" things for us are things that have "always been there" in the eyes of the Pokemon World. The Regional Pokedex was formalized as one of these retconning concepts, meant specifically to document Pokemon that are in-lore native to the location at the time. Kanto and Johto received degrees of separation despite being neighbors, hence why you didn't see future Kanto Native Pokemon show up in the FRLG or LGPE Regional Dexes.
And yes, this whole retcon procedure also includes matters such as making it so those "100 new Pokemon" were no longer new, likewise retconning when Eggs were discovered so that there wouldn't be lore conflicts with you being able to receive a Togepi Egg and breed Pokemon in FRLG.
There's a lot more obviously, but most of it boils down to "you couldn't do this in a prior game because you didn't have access to this item/move/mechanic/etc.", which is primarily why cross-gen evolutions are so reliant on gimmicks or item bloat, sometimes to comical results (i.e. implying that only Hoennians, Kalosians, Alolans and Galarians are capable of caring for their Pokemon genuinely enough for Eevee to evolve into Sylveon for it to be recognized in a Pokedex where Eevee is included).
