2020 had Animal Crossing New Horizons, which is a second killer app for the Switch (or third, f you count Mario Kart 8).
The main reason is the hybrid nature of the console. Handhelds typically peak later than home consoles, and Switch is behaving more like a handheld. I suspect the reason for the early peak for Nintendo home consoles is because if 5 people live in a household, and 1 wants one, they get one, and then all 5 people have one; so if the other 4 want one, they already have it. With handhelds, if 1 person in that household of 5 wants a handheld, then that person gets one, the other 4 will buy there's later.
A hybrid Switch might actually have an advantage on handhelds because it can switch into a console for others to play who wouldn't normally play, which might serve to widen the demand by increasing the overall exposure to the console. Many people, especially young people and office/commuter class people are going to want their own Switch eventually for its handheld purposes, and usually young people live in households, so if there are 5 people sharing one Switch, that might turn into 5 people who want five Switches a few years down the road, especially if the younger people are going off to school.
Anyway, that's my theory based on what we've observed and the hybrid nature of the Switch.
I describe myself as a little dose of toxic masculinity.
