Because its a console which also is a mobile device. It not only comes with one controller, but, depending on the game, the JoyCons allow two players to share them. Without buying additional hardware. They also happen to be complete Wiimotes with motion tracking - something you had to buy extra before.
And of course, it is the best and most powerful gaming hardware made by Nintendo. A PS4 is cheaper and has more GPU power, if you don't mind it being a big box. And it doesn't play the Nintendo games.
The hardware actually looks like the ideal form factor for me right now (I have young kids, so it's hard to sneak to the den for those marathon gaming sessions), but honestly the thing I really care about is the software.
Because they have legions of rabid fans who not only buy their products regardless of quality, but act as an army of apologists for them. The result ironically, is that unlike Sony's harsh lesson learned around the PS3, Nintendo has no such backlash to fear.
The worst they can do is just own the portable market, and sell a few iterations of a console to their diehards, which are numerous.