Sakurai has given an interview with Famitsu magazine in which he delved into several aspects of the new Smash Bros, including, notably, the absence of two-time veteran Ice Climbers.
"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."
The Icies may be gone, but they live on in our hearts. When you really think about it, the decision to cut them for the sake of quality in the game was probably really hard to make. We’re sure the team did what they could.