Because the first steps that one needs to start hacking their wii cannot use anything else. Maybe one could hack the wii with a usb drive if the wii actually was able to store and read data off it in the first place.
TL;DR usb drives cant be read without hacks, SD cards already accepted.

Because neither the Wii itself nor any of the games use a flash drive, so unless it's hacked you can't get it to read any data off of it to do something with...Wii can't I just use a USB memory stick?