Super Mario 3D World didn't sell too well, because people had already played 3D Land and other 3D Marios, and the game had nothing really original or remarkable. Splatoon on the other hand, was a fresh, original game, and everyone wanted to try it out.
Now, the situation has changed. Splatoon 2 is just another Splatoon game, while Super Mario Odyssey is different from all previous Mario games.