I challenge the premise that the regions don't communicate, but first we need to talk about parallel universes...
In all seriousness though, each major revision of the base game has brought with it some minor retcons to the story, as well as running some stories in parallel to previous games.
There is an extensive chronological history of events that happen before and during the games. Some entries list NPCs and Professors moving or traveling to different regions for study purposes. Some protagonists (and their parents) also travel into their new region before beginning their journey, such as Brendan/May in Gen 3, and Sun/Moon in Gen 7.
In fact, Generation 2 confirms that Professor Oak is friends with Kurt (the Pokeball maker from Johto's Azalea Town), as well as Professor Elm, who actually used to be Oak's assistant. At Kurt's place there is even a picture of Kurt with "a young Prof Oak", and on Elm's lab computer you can read an email from Prof Oak to Elm.
There is also the addition of the train line between Kanto and Johto, as well as the ferry, allowing for ease of transport and commuting between the regions.
Regarding the Pokedex, note that Professor Oak never says that there's only 151 Pokemon in the world:
...This world is inhabited by creatures called Pokémon! For some people, Pokémon are pets. Other use them for fights. Myself… I study Pokémon as a profession.
Sure, we know that there was only 151 Pokemon in the Pokedex, but that was because it was a "Generation 1" device, a first attempt at an encyclopedia:
On the desk there is my invention, Pokédex! It automatically records data on Pokémon you've seen or caught!
Indeed, if we look at the Pokemon PC Storage solutions, most of the regional technicians were in close contact:
Your storage system offers more convenience than mine. It has a lot of user friendly features that make it fun and useful too. It makes me proud that I played a part in its development. Here's hoping that you'll continue research in Storage Systems.
- Bill, to Lanette as an email in Lanette's lab
Bebe (Generation IV) based her storage design off the previous iterations:
It was she who developed Sinnoh's Pokémon Storage System based on the previous designs of Bill and Lanette...
and she appears to be friends with Bill:
..After the player obtains the National Pokédex, she will give him or her a level 5 Eevee in Diamond and Pearl. This Eevee came from a good friend in Johto (Bill) before being passed onto the player.
Bebe - Bulbapedia
This is only a few examples, but I believe it helps prove that the major game regions do in fact communicate, share ideas and collaborate on research. While early games may have had a limited number of Pokemon available, I think we can just chalk that up to the real-life development of the games - in universe, all the Pokemon were always there, just waiting to be found, caught and catalogued.
After all, isn't that the whole point of giving a bunch of kids Pokedexes and sending them out on an adventure in the first place?