As everyone knows, the GameCube controller allowed for a certain amount of versalitlty when it came to Smash Bros. games. With the C-stick option, players could use it for quick smash attacks, which was a great thing to have in tricky spots. But with the confirmation that the New 3DS won’t arrive in most regions until 2015, people were wondering if the Circle Pad Pro could take that spot in the meantime — the answer according to producer Masashiro Sakurai is “no.”
According to Sakurai, “Supporting the CCP requires the use of some of the CPU’s processing load. From the information received during the development of Kid Icarus: Uprising, this is a quite large load at around 5%. Kid Icarus: Uprising and Smash both use the maximum capability of the device, so we had to abandon the idea of CCP support. With the New 3DS, we were able to support the C-Stick as the processing power is increased and it is not a peripheral device.”
So there you have it — the 3DS isn’t powerful enough to support it — though he didn’t fully explain why it did work in Uprising. My guess is that it didn’t actually use all of the portable’s power, whereas Smash 3DS definitely does.