* It's a Nintendo handheld. It was always going to do well.
* It's not looking like a Wii branded accessory.
* It took the best games from the WiiU and re-released them and sold bonkers.
* Price.
For me I think the biggest reasons why the handheld sold so well is because it appeals to everyone, moms and dads are more likely to buy one for each child on a long road trip for example to keep them entertained in the back. - Happened with me and my siblings back in the 90's with the Gameboy.
This is in stark contrast to conventional consoles that sit fixed under a TV, you only need one for the entire household.
--::{PC Gaming Master Race}::--