Good games tend to sell well
See wii sports

For me it's because I don't have to shut myself in a room to game now. You're telling me I can play *insert game* on the couch while my wife is doing something else? Excellent! I can move to and fro, and therefore I am buying/downloading games I wouldn't normally buy. I've got over 35 hours logged on that League of Legends clone on Switch, my goodness it's ridiculous. I'd never download that game to play on a home console or PC, but on the go it's a blast!

A lot of those sales were due to being bundled with the Wii.

My guess is that it appeals to more than core Nintendo fans.

Gotta second this on a basic level, the software on the Switch sells simply because it's good software, sadly there was a lot of it on the WiiU but that hardware was a massive barrier which people weren't willing to jump over to get at the software.

The Wii U had a terrible name, and honestly I think the price point killed it.  The console was $300, but half (roughly) was due to the controller....  and the Gamepad was terrible.  Nintendo could (and should have) gone at $200 with a standard controller and called it Wii HD.  

Brand confusion really hurt it as well, 2-3 years after it came out I had people asking me about getting the gamepad for their Wii and just plain not understanding that it was a new system with its own library of games.

Realistically the WiiU was what they wanted the Switch to be, but the hardware wasn't really there in 2012 or well around 2010 when they would have started working on it, from a functionality point of view the system offers the same type of play experience as the Switch but just horrifically hampered by the hardware available at the time, the 640p screen on the gamepad looks very unclear as well after you've played some Switch games.

I guess one great thing came from it if you look at it like that though... it was a prototype for the Switch and it showed Nintendo exactly what fans didn't want.

I still don't regret getting a U. Wind Waker, twilight, Bayo 2, Mario 3D, Pikmin 3 and a few others were stellar games.

Lego City Undercover, Mario Maker, Indies for days, Mario Kart 8, Mario Kart 8, Mario Kart 8, I could go on.

maybe not having to buy a handheld and a homeconsole seperately as well as software for each device has freed up money to be spent on more games i donno lol