Wii Sports was a fad. The millions of casuals who bought Wii's to play it have zero interest in a WiiU, seeing how the Wii's are collecting dust.
the Wii was a system that succeeded on fads, first it was Wii Sports, then Guitar Hero (GH3 did stupid numbers for it if I recall), then fitness 'games' took over, while actual games like Mario Kart had a shot for awhile, and then Just Dance and Zumba resurrected a quickly dying Wii for a bit, which kept the Wii's heartbeat going just long enough so physical toy games from Activision and Disney could keep the system relevant at all.
its a formula that can't be replicated. But for years, that little white box was hard to keep down.