Because Nintendo likes to kill off consoles early.
As someone already pointed out, the Wii was selling very well and Nintendo just happened to announce WiiU and not release any more big games for Wii. Wii sales then fell sharply.
It all depends on Nintendo. If they start releasing Switch 2 in 2022 and their games won't support cross-platform, then Switch might not get to 100 million sold.
Wii sold only 5m in all of 2012, which was the year WiiU came out (November 2012 to be exact).
IMO it probably will reach 100m but not more than 120m.
Another context. Wii and DS. Nintendo has two consoles to take care of: handheld and stationary.
Nintendo claims Switch will have a long lifespan.