Galaxy failed to be a killer app. Notice how the term "killer app" isn't being thrown around so much anymore; it couldn't be associated with this cycle's high-profile HD games. PS3/360 killer apps failed to propel their consoles to the levels people hoped for, while Nintendo redefined what REAL killer apps do via Sports/Fit/Kart. 3D Mario did not propel Wii to success, Wii simply provided a habitat for 3D Mario. On the contrary, 2D Mario does propel Wii (and captures the attention of Japan). Thus, 3D Mario can be deemed irrelevant, like a pretty successful 3rd party game -- certainly not important enough to truly push sales of the machine.
2D gaming has its charm. It's honest-to-goodness classic feel-good video-gaming that doesn't require so much extra brainpower to grasp, and gameplay easily accommodates display monitors of nearly any size (particular small, personal screens). Decades of gaming were played on tiny, yet personal TV sets (and PC monitors were even smaller).