According to a recent Tweet from The Pokémon Company (translated by Gematsu), Pokémon: Let's Go Pikachu and Eevee have broken the Switch's first week sales records on the worldwide chart. In total, 3 million units of the game have shipped in the first week. Those sales numbers have made Pokémon Let's Go the biggest Switch launch to date.
Pokémon: Let's Go is a hybrid between Nintendo's mainline Pokémon games (particularly Pokémon Yellow) and Pokémon Go. The concept itself is pretty adorable, and having an Pokémon RPG on the Switch is something fans have wanted since the console launched last spring. Add in the nostalgia factor of basing the RPG on Pokémon Yellow and combine it with the adorable art, and you've got an instant hit.
Seriously, I'm still in the middle of playing this game and I cannot get over how cute it is.
There's also the bonus of allowing Pokémon Go players to get their hands on Meltan by transferring Pokémon between the two games.