If I recall correctly, Mariokart 8 for Wii U was also insanely successful. Couple the quality of the game with such a sparse selection on retail shelves, and I'm not surprised in the least.
I guess i'm part of the problem if you aren't for Wii U ports of games, but I only paid $25 for it after offloading my Wii U copy. Won't lie, I think the gameplay is so good on this version of Mariokart that I find myself hoping they just release more DLC tracks throughout the Switch lifespan, rather than release a new sequel (Mariokart 9).