It's called "not making games people want to play." Demand for games was still there. Wii Sports Resort was a huge hit. Wii Fit Plus was a huge hit. But Miyamoto's shortcomings began to show themselves again in Wii Music, and Animal Crossing. And you had two developer vanity products (products that were made to satisfy the demands of developers with little thought to what customers wanted) in Mario Galaxy 2 and Metroid: Other M.
Wii Music was a cornerstone of their 2008 lineup. And it was a weird game. (Tennis isn't weird. Fitness isn't weird.) It was a huge risk, not because it was expensive to develop, but because they had so little else planned for '08.
How much do you think Miyamoto had to do with Animal Crossing? And when Wii Fit was released, Fitness in a gaming context was very weird.