All digital titles have something called an apploader which (on software made with the official SDK) is basically a common piece of code responsible for loading the actual program that was written;
for licensing reasons homebrews couldn't directly copy the official one so they had to write an equivalent (well before the WiiU was made, of course) and the commonly used one* turned out to be not that equivalent to the point of also work on Vwii (compare to the "TR" wiimotes not working on older Wii homebrews, also because of inaccurate drivers)
* the one used in the HBC obviously works on both Wii and Vwii; don't ask me why they didn't just use that one (probably because the HBC was closed source until relatively recently)
