Pokemon: Let's Go, Pikachu and Let's Go Eevee take place in the Kanto region (the region originally introduced in the first generation Pokemon games, Pokemon Red, Pokemon Blue, and Pokemon Yellow). Nintendo says that only the first 150 Pokemon (plus Mew, for anyone who purchases the Poke Ball Plus) will be available in these new Pokemon games.
It is currently unknown as to whether these games will eventually be updated to include later generations of Pokemon characters to capture. It isn't unreasonable to speculate that later generations of Pokemon will eventually be released in future updates for these games, as they have been for the related mobile game, Pokemon Go. Only time will tell.