The illusion of simplicity. Even though pretty much every tech hardware nowadays is a complicated hot mess, it's important to not to confuse the consumer with it.
DS was
the brain training machine. It has a bunch of buttons and stuff, but it's main purpose was holding like a book and play with a stylus. Many consumers only used it for that.
The Wii had a nunchuck and alternative controllers. But, for most people it was a giant motion controller to play Wii Bowling.
3DS added counterproductive tech: 3D doesn't work in portrait mode.
Wii U added a counterproductive tech: you can't swing a bowling ball with that giant tablet.
Switch, while mostly a success for core gamers, still did something 'backwards'. It added back a simpleton mode: single joy-con mode. While it's hard to imagine and easy to forget while being on this forum of enthusiasts, for the majority of the world Nintendo is still mostly a family friendly console, with lots of people of all ages playing. Mario Kart with a single joy-con is not threatening to play.
Though, Nintendo did cast a gigantic wide net and it went a completely different route than what they were thinking they were pushing. Remember 1-2 Switch? That was their attempt to capture the casual crowd.
What happened is: holy shit, this is the most powerful handheld to date and it plays freaking Skyrim. They underestimated how much gamers just like games. But the marketing, I believe ran by NoA, saw the potential much better and totally nailed the message.
Now buy some third party games, darn 1st party-only gamers

I understand families buying family-friendly games because they're safe, but this is supposed to be a group of enthusiasts...