Let me explain why usb loaders were homebrew only until the wiiu (in regards to the nintendo line of of consoles) the wii had no means of installing retail games on it's system. The wiiu however already has that. You can install games into a usb flash drive if you wanted to. Simply by plugging it in, have the wiiu format and your done, a usb loader, grab cfw for signpatches, install a game onto the usb and your done (wup installer gx2 gives options to install into usb or internal nand). There's not much more to that.
with the wii again that was impossible, so people had to implement ways for a disc to be read over usb. But because people can just install titles to either the usb and the internal memory on a non modded wiiu already. (because Nintendo gave us that ability) there is no other loader other than the wiiu one. All is required is signpatches for eshop games and dlc. and a entry point, which is obviously web browser or a ds vc exploit