What do you think the in-universe justification for the national pokedex is? like why is kanto first, then johto, and so on. Is it because Oak created the pokedexes?
I think maybe the better question is “why is there an order at all?”
They’re not physical books; they don’t need to be printed, so there’s no need for the entries to actually exist in any sort of canonical order. The user can just ask for one specific entry, or for a list of entries arranged alphabetically, or by type, or by geographical distribution, or whatever. In-universe there is no obvious reason why, for instance, the Pidgey line should come immediately after the Weedle line. And then, of course, the one clear ordering principle – the fact that evolutionary families go together – is then violated apparently at random (again, from an in-universe perspective) by Pokémon like Pichu or Kingdra.
The fact that Kanto and all of its Pokémon come before Johto and all of its Pokémon is really just a manifestation of a much more fundamental nonsense in the way the Pokédex works. In fact, it’s not even as straightforward as “Kanto before Johto” because there are a few Pokémon that don’t even obey that – Togepi, as far as I can make out, is native to Sinnoh, not Johto, as Sinnoh is the only place we’ve seen them wild; likewise, Eevee seems to actually be a Kalosian Pokémon.
Of course the real reason for the existence of the rigid numbering system is that it’s part of the “collector” mindset that’s fundamental to the games. That comparison might suggest something about the historical aims of projects like the Pokédex that were run on pen and paper; Professor Oak certainly isn’t a “collector” of Pokémon and wouldn’t, I think, endorse a view of Pokémon as collectibles, but he could be building on a body of scholarship that had traditionally been about a philosophy of Pokémon training which was more… elitist, controlling? Not sure there’s any way to get at the specifics of why the regions or individual Pokémon are in the order that they are, though.