Because Nintendo handhelds have always sold about 70 million, minimum. (The 3DS will get there eventually, in fact, it'll do it before the year is over and could end up at 75 million.)
The Switch is the 3DS' successor, as well as the Wii U's. Thus, it's getting every single bit of 1st party support that would previously be divided across 2 systems.
Thus, going from a combined figure of 75 million (3DS) or 88-90 million (3DS + Wii U) to just 45-50 million, despite getting all the signature Nintendo 1st party support (Mario, Pokemon, Zelda, Animal Crossing, Splatoon, Fire Emblem, Metroid, etc.) + decent to great 3rd party support would be a colossal disappointment and failure. That would be Xbox One levels, which the Switch is absolutely demolishing at the moment, launches aligned.
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PlayStation/Xbox/Switch: 2022 Edition
PlayStation/Xbox/Switch Hardware Battle: 2021 Edition!
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020
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PlayStation 4: 2015 vs. 2016 vs. 2017