"We already have well over 800 Pokemon species, and there's going to be more added in these games. And now that they're on the Nintendo Switch, we're creating it with much higher fidelity with higher quality animations...We knew at some point we weren't going to be able to indefinitely keep supporting all of the Pokemon..."
Masuda, 2019

It's over. I mean, it's completely over. I don't think that many of you actually know what is happening, THERE WILL NEVER BE AN UPDATE, PATCH, DLC OR ANY FUTURE GAME WITH ALL POKEMON AGAIN

This is not a mistake or they rushing to finnish the game. It's a decision.