Attractive Hardware, that’s literally it!
edit: to be more clear, hybrid model that just works for everyone casual/hardcore gamers alike.
Everyone wants games that they can play their way and NSW is just that minus the extra pretty graphics
It's not just that. I mean, the Switch being a great hardware is a big factor, but it's not the only one.
Great marketing, one that understands it's public and it's effective with it's messaging.
While not exactly a concenssus, most Nintendo IPs are getting it's best reviewed entries, or the best one in decades, on the Switch.
Dammed demand of Nintendo games. Wii U was a failure and many wouldn't want to touch it even with a 10 m stick, and the Wii didn't exactly satiate the demand for more "hardcore" games from Nintendo(The nostalgia of games from the NES, Super Nintendo, 64 and Gamecube to some extend)
And the momentum created by that first year. I think people keep underestimating the avalanche that was BOTW + MK 8 + Splatoon + Mario Rabbids + Mario Odyssey + XC 2, and the smaller Nintendo games and indies.
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.