The Ice Climbers duo, Nana and Popo, have been a fixture of the Super Smash Bros. series since 2001's Super Smash Bros. Melee for the Nintendo GameCube. Plucked from obscurity out of the lackluster Nintendo Entertainment System title Ice Climber, the pair exist in Smash as a single entry on the roster, but fight as a team. Now that teamwork has cost them a place in the new Super Smash Bros. games for the Nintendo Wii U and 3DS. Nintendo has chosen to keep the character roster identical across both platforms, and that decision combined with technical challenges has led to the elimination of the Ice Climbers. According to developer Masahiro Sakurai via Famitsu translated by Nintendo Everything, Nana and Popo did not take to the 3DS so easily.
[R]egarding the roster, Sakurai mentioned that even though many characters are included, there are some who worry about those who didn’t make the cut. He specifically addresses the absence of the Ice Climbers as well. It seems they were running in the Wii U version, but due to the required hardware power, the team couldn’t make them work in the 3DS game. There wasn’t as high a priority given to series that are unlikely to have another installment at this time.
That's a sad end for the duo, and I wonder if they'd have made the cut if they were more popular. Would the development team have spent additional resources to get them working on the 3DS if they were all-star captains of the series? If Ice Climber U was due out next year, would things have ended differently? They can be a difficult challenge to use for those unfamiliar with how they perform, and they're not the only characters to go missing this time around; Kat Bailey at USgamer has put together a tribute to other absent friends such as Solid Snake, the Pokemon trainer, and Wolf O'Donnell. None of the cut characters impact the way I play Smash, so I can't say I'm mourning their loss, but I sympathize with players who wonder which character to use primarily now. If Mario or Link were somehow unfathomably removed from the game, I wouldn't know what to do.