Because it's geared towards kids. Kids have a lot of time to play video games. Kids also need to be entertained constantly, so the Switch's portability is icing on the cake. Kids also crave couch multiplayer, which the Switch offers a lot of. And kids also grow up. Nintendo has been focusing on kids for the past 35 years, so now a lot of those kids are adults who have their own kids, which has helped the Nintendo console become the perfect "family" gaming system.

The other systems have allowed Nintendo to take the kid/family market, while they've shifted their focus to more serious gamers. The "big 3" systems are all very popular, but Nintendo has captured a special segment of the market.