He said that it isn't that easy to support both GC-Pads and USB-controller. So he had to decide between this two options and chose to only support GC-Pads. Nonetheless he said that it is one of his goals to support both controls in the Wii-version, but this issue has rather low priority to him. So it will take some time before he tries to implement it.
I also asked if he could make two Wii-versions, one with GC-support, one with USB-support. Because it's just a homebrew app it would be possible to have both versions on the Wii. But he answered that he has already four version (WiiU USB, WiiU SD, Wii USB, Wii SD) and don't want to make more.
So it seems that owners of Family edition Wiis need to have patience.