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.
Huh? This is exactly what the OP is saying haha. "probably" will reach 100m?? It will probably reach 100m NEXT YEAR! It should end next year right around 100m. And the Switch 2 will still be multiple years away at that point. 120m is like the guaranteed bottom at this point. Like, if Switch died HARD and very soon it would still sell 120 million at this point. Again, this is what the OP is talking about, why do people still say stuff like this?
Maybe it's the talk of the new Switch model and some people seem to get confused and for some reason think it's a Switch 2 and not just another Switch model that is coming out soon. Switch 2 is yeeeaaaarrs away, 3-4 years away at this point. And in just over one year from now Switch should hit 100m. That's a hell of a lot of time to sell above 100m before the Switch is even replaced.
If Nintendo replaces Switch a little on the early side, like say holiday 2023, Switch should be looking at over 130m lifetime sales. If they replace it at what I think would be the perfect time, Spring 2024, it'll probably end up around 140 million. If they keep putting out good first party games for it in 2024 and don't replace it until holiday 2024 and the price is all cheap by then, then it could hit 150 million (49+29+24+17+14+9+5+2+1 = 150).