The Wii had several droughts even when it was selling like gangbusters.
Those droughts (like fall 2008, the infamous Animal Crossing/Wii Music holiday) went largely unnoticed because Wii Sports/Fit were such long tailed monster phenomenons.
It's kinda like a sports team that has two superstar players -- they make everyone else look good too and can mask a lot of a team's weaknesses.
Same thing with the DS, after 2007 or so they didn't actually need to support it as much because Brain Training/Nintendogs/NSMB had already done most of the heavy lifting and could carry the console for the rest of the product cycle with intermittent support.
The problem with the Wii U is really not that it has fewer games than the Wii ... it probably actually has a better overall library top to bottom than the Wii did at the same point in its life actually ... it doesn't have those "superstar" titles like Wii Sports that broke out and carried the Wii for years.
Also I think the Wii benefitted a lot from late gen GameCube titles that basically were just directly ported to the Wii with added controls. If you remove or delay Zelda: TP, Super Paper Mario, and Fire Emblem ... which were all GameCube games to start with, the lineup for the Wii early on is much more sparse.
And while its true the Wii U has some projects that started off as Wii games, like Pikmin 3, they definitely had to be retooled to be Wii U games because of the large difference in hardware horsepower.
