People give all the credit to COVID and the shelter in place. But I think Animal Crossing Pocket Camp for smarthpones was a factor just as decisive. When the mobile game arrived millions of people who had never touched an Animal Crossing game in their live suddenly found themselves enjoying a watered down version of it. Then Nintendo starts promoting New Horizons for the Switch, which completely blows away anything Pocket Camp could offer. If I were a Pocket Camp player and I watched the Animal Crossing Direct... No question: I would definetly want to get my hands on it. PC is a joke compared to NH.
Iwata nailed it. Use smartphone games to grow the sales of their console games. It happened with Fire Emblem, it happened with Pokémon and it happened again with Animal Crossing.
Add to this the COVID situation, the massive success that is the Nintendo Switch and that the game recieved excelent reviews... and you have a bomb.