There is actually only one reason why Switch is a huge success - Nintendo built a "cool" product that sells to the mainstream audience. I mean, Switch is probably the best Nintendo console in like 20 years for so called "hardcore" gamers but it also attracts a lot of casuals (just like PS4, another successful console this gen). Switch is the "next big thing" in gaming technology. It's as simple as that.
Just to clarify - I'm not talking about only technology of Switch, but the about the mix of technology and great games, that appeal to a large audience.