In the 2014 title, the game's director, Masahiro Sakurai, planned to have the Ice Climbers in the game. The program team had managed to get them to work in the Wii U version, but not on the 3DS. Having a character only work on one system wasn't gonna cut it, since Sakurai wanted the roster to be the same on both versions, so the Ice Climbers were cut. This problem of console parity was big for many PS4/Xbox One/Wii U/PC in mind next gen games that had ports for less powerful hardware such as PS3/Xbox 360/PS Vita and 3DS, as other big games such as Destiny 1, Watch Dogs 1, Mighty No. 9, Metal Gear Solid V, etc all got affected in similar cases becuase of less powerful hardware being involved. That last from late 2013 when the 8th Gen was in full affect all the way to late 2016, before the Nintendo Switch launched and that took over the PS3/Xbox 360/PS Vita and 3DS as the new highest standard lowest common denominator bar in exchange.
Saving the Ice Climbers
Many petitions have been put up to get the Ice Climbers (or just Popo with Nana as alternative skins, like Wii Fit Trainer and Villager) back in as DLC. With the Smash Ballot created on April 1st, fans can now enter them and hope for the best.
Unfortunately, Sakurai ignored all these cries, and released the final DLC pack consisting of Corrin (from Fire Emblem) and Bayonetta as the sole winner of the Smash Ballot.