Breath of the Wild + Portability = case closed.

Good marketing, nice concept and one of the highest rated games at launch. That's a good recipe for success.

Because it gives Nintendo fans what they want, the real question is how much it can capture outside of that market.

Because it's a cool device with some good games that you can play on your tv or in a portable manner with no hassle.

Breath of the Wild in particular is excellent game and aside from WiiU emulation, the Switch is currently the best way to play that game.

Because it's fantastic? :o What more reason do you need?

Mr. Miyamoto sacrificed the world's last two-headed Amur Leopard on the altar of Mercury using the jewelled dagger of prosperity.

Nintendo staff, dressed in their finest designer suits, then bathed in the blood, and drinking as much as they were able.

The only reason Switch is even relevant right now is because of the "Nintendo" name attached to it. It this were a MS system, it would not even see half of the sales its getting.

Zelda and other Ninty exclusives, decent third party support and some HW features that users consider useful and desirable enough to justify the price.
Plus being a very well-thought-out project after a disaster on home consoles and a portable below expectations, basically it's what both Ninty fans and simple gamers that aren't Ninty haters expected from Ninty.
I'm not a Ninty fan, and I still find NS expensive for my tastes and my wallet, but I can tell an excellent product when I see it.

"Why Do You Think Switch is a Sales Success Right Now?"

For 1 simple reason: Zelda BOTW, which has been totally overestimated, as always. When this game won't sell anymore, the switch will become a bigger flop than the wiiu. Maybe even when their online will become payable.

You know, Wii U also had "Nintendo" name attached to it.