Has there ever been an in-game reason behind why new Pokemon are being "discovered" each generation? I remember a little bit of stuff from Gold/Silver on the subject, but not much. You could argue there was a lesser amount of technology/internet communication between regions in the early games, but that would make no sense by this point. Almost every Pokemon should be discovered now (aside from some legendaries), so why does each generations' pokedex not include every Pokemon? The most glaring examples of this are new evolutions (or pre-evolutions) to "old" Pokemon. How could some of those Pokemon not have been discovered until a later generation? Surely someone in Kanto must have had an Eevee that would have evolved into something other than it's original 3 forms.
In the end, this all boils down to "it's just a game". But I'd like to think there is some logical reason behind it.
If you had to explain in-universe this phenomenon, the only thing that works is lack of communication. All else notwithstanding, that Dialga/Palkia statue in Eterna City predates Oak. That's just one such example.
Go to Gamestop, buy a Steam card, and use it to purchase Ys Origin. You won't be disappointed. Ocean should not derail the train.
The (region) Dex is meant to record Pokemon associated with that region only, while the National Dex (in theory) would record data from foreign Pokemon, but your character is unable to see/acquire them.
"Murder of the living is tragic, but murder of the idea is unforgivable." - Janus, speaker of the synod