I guess the reason it sold so well goes beyond the pokemon fandom

1.- The switch is one of the most popular consoles right now, it's basically this gen's gameboy. Handhelds would always be very popular with parents who buy a videogame to their kids but are not fully commited to buying them a console.

2.- Practically all millennials right now are at working age, everyone who was a pokemon fan and couldn't afford the games when they were kids will buy them now.

3.- A lot of pokemon go normies own a switch, unlike the 3ds, one could argue the switch is even more popular with the casual audience, thus boosting the sales of Let's Go and Sw/Sh

I can't really think of any other reasons for the games selling so well, i've been a pokemon fan from Pokemon Yellow to Pokemon X and I know i'm not the only one who didn't buy the new games, yet they sold better than 75% of the other games.