Most analysts seemed to place the switch as a "Nintendo home console" and used the declining data from N64, Gamecube and WiiU to paint the Wii as an outlier and the Switch as irrelivant.
This is in contrast to the reality where 3 of the last 4 devices released by Nintendo in the past 15 or so years have sold >50 Million units. If you go back to the GBA its 4/6 in around 20 years.
Turns out the Switch's potential numbers are more closely following that trend then console doom and gloom naysayers.
At this point, the only thing I can think of is that the machine's visual design was great and the ads worked well.
It has an extremely strong launch game probably the strongest in a decade. People buy consoles to play games. How long Zelda manages to carry it, remains to be scene but keep in mind this a game that was solely brought to E3 and still managed to turn said E3 into a success.
I cannot think of any other reason as to the Switch being Wii~levels of success.