By default, USB is encrypted. That would give homebrU a place to store files, but a computer couldn't make heads or tails of it. We need the unique per-console OTP keys to allow a computer to read a WFS formatted drive. The files could be saved on an unencrypted USB stick, but that would be a lot of extra work for someone to rewrite the USB IO, and (probably an IOSU exploit to even allow that). The Wii U does have built-in support for unencrypted SD cards, but sadly the browser lacks the permissions to use those features. That's why we need an IOSU exploit.
EDIT: You got me again,
NWPlayer123!