Think you're ignoring a lot of context; NSMBW outsold Galaxy because the game came bundled with the Wii console, and the Wii was an absolute hit with the casual audience. Galaxy and Galaxy 2 outsold every other 2D platformer that wasn't bundled with the Wii at the time, including Donkey Kong Country Returns, and Return to Dreamland. Super Mario 3D World also came bundled with the Wii U console, and sold slightly more than NSMBU which also had a bundle, and 3D World outsold Tropical Freeze, and Yoshi Wooley World. And Super Mario Odyssey has outsold Kirby Star Allies, Yoshi's Crafted World, and the Switch ports of NSMBU and Tropical Freeze as of this writing. So if anything, it's reversed; with the exception of NSMBW, which as I said came bundled with the Wii to give it a boost, every 3D Mario platformer has outsold every 2D platformer on the same console. So as I said, there's a higher expectation on 3D platformers than the 2D ones.