In Sakurai’s bi-weekly column in Famitsu magazine, he revealed why the Circle Pad Pro attachment for the original 3DS/XL will not be supported by Super Smash Bros. 3DS (thanks to StreetsAhead for the translation).
(What is the reason that the game supports the New 3DS’ C-Stick, but not the Circle Pad Pro?)
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.
It seems that the lack of support comes with good reasoning, but it’s still disappointing to hear that any future support is unlikely.