3 reasons that made Nintendo Switch a succes

[Slownenberg]

Basically:
1. Hybrid system is a very appealing idea (play anywhere, console level games on a portable)
2. Zelda. BotW was the Switch's version of Wii Sports, though obviously a very different type of game. The killer app of the system that was available at launch and generated huge positive word of mouth.
3. Huge 1st year. Beyond Zelda they had Mario Kart, Splatoon, and Odyssey which again like Zelda was an incredibly well received release of their other main franchise from the NES.
4. It has become the best way to play indies

I don't think marketing is that big a reason. Nintendo commercials and the like have never been a big seller of the systems. Word of mouth is what sells Nintendo. Wii had that, Switch has that. A friend has Switch, other people play it, and are like oh I gotta have that, and word spreads. Switch's marketing is baked into the fact that its got the awesome hybrid design and got must-have versions of the great Nintendo games everyone loves.