Simplicity, Price, Games (Exclusives), and Marketing. Nintendo just hit it out the park with all these aspects. On a more personal note, I just appreciate the ability to play such high quality games anywhere and any way I want to. It's really been a release, being able to be in charge of how I game. And it shouldn't be understated the arsenal of 1st party games Nintendo has yet to release and the strong support of indies, and decent 3rd party support.
The Switch really is, I think, a console for everyone. While the Wii U was obviously directed at kids. My parents are even talking about it and they don't really play games. It's just a magical device, and it really shouldn't be. In essence it's just a tablet with HDMI out, but it is a fantastic device.