cause they named it wrong.The Wii is really the only time they managed to do that after rescuing the western home console industry in the mid-80s. Very few Wii owners from that group went on to buy a WiiU, for whatever reasons.
Older generations who bought the Wii to play with it for themselves as well as with their children really helped propel sales for the Wii. The motion controls and Wii Sports and dance games made it more successful than Sony and Microsoft's consoles.
The Wii-U could've continued capitlizing on this if only they had changed its name to something different and advertised it more clearly. Just the name itself was a royal fukk up. 3 years ago my mom wanted to get a Wii to play Zumba I asked her to get the Wii-U instead, she came back with the Wii, I was mad af cause I wanted to play Ninja Gaiden 3 Razor's Edge. She said oh well they both play Zumba its the same thing.
I wouldn't be surprised if that same demographic will think Switch is the successor to the first Wii, sales would be high af.
