Nintendon't is supporting it, because the controls (control emulations) are implemented in nintendon't. You just code it, and it's working. But retail games are already done, and unless the dev have implemented it, it become a bit harder to change that fact in the future.
But sure, everything is possible, it just has to be done. But at first, you have to understand how the WiiU and Wii are handling the system functions:
The WiiU and Wii have a different approach to use the system functions. Dynamic linking vs. static linking.
On the WiiU, all system libs are loaded on run time (The .rpl files!), this mean dynamically. When a game needs WiiMote support, it loads the padscore.rpl and has access to the KPAD-functions. What HID-to-VPAD does it just hooking into the
functions of the .rpl's. Very simplified: The game is asking for system function XY and we're given them a patched version of XY (In fact it's done differently, but the result is the same). Because each game is taking the function from the same place, this can be easily done, and the functions only need to be patched a one place. The WiiU also has a buil-in HID lib, which can be used easily.
On the Wii, the system libs are linked STATICALLY. Every Wii game has a version of the functions directly in their executables. All games run on the bare metal, so there is no OS, this mean they can't ask the system for function XY. They have their own copy of the function in their .dol that they are using. Every game may have it in a different place and may even have a different version of it (When it's compiled with an older/newer SDK function). In addition you don't know where the function is in the .dol (on WiiU side you have the neat findexport function), but at least no convient way to find it.
There IS a tool for the Wii that handles with all this "problems", called brainslug by Chadderz:
https://github.com/Chadderz121/brainslug-wii.
This means it can be done! But at first, someone has to implement it. You need to find the "signature" of the control-function in the Wii Games, that indentifies the function in the executables, which then can be patched. This also has to fit all versions of this function. Once you've done this, you can probably copy the nintendon't hid-handling code for using the GC-Adapter, do some magic, and hopefully it's working magically.
tl;dr: It's just way harder on the Wii, because all games have their own copy of system functions compiled in. This means all games have to be patched individually (or you use the neat brainslug app for patching). + no HID library provided by the system, like the WiiU.