Personal note: this makes sense, but it either assumes some incorrect things about how Wii works or assumes some very, very specific things about the implementation.
Just saying that would be rude, so let me explain. Similarly to how Windows works, Bluetooth (which covers Wii Remote, attachments and U Pro Controller) is split on Wii into two separate subsystems: driver for antenna module and so-called stack, which as long as I know interpretes the commands.
Now on any regular sane system things would work like this: OS contains the driver and the stack, converts the messages to controller data, then exposes this through some sort of API. However, Wii's IOS (which is not really OS, but this doesn't change much for the purposes of this particular problem) contains only the antenna driver. Every Wii game contains its very own (library) implementation of Bluetooth stack. I have two theories why they did this. One is that IOS was too slow to handle controllers or designed too late, another is that they wanted easy support for weird Bluetooth controllers.
What this basically means, adding support for new controller type is a mess.
Now, with them reselling selected Wii games, they can patch/recompile them if that helps to add GamePad support. On the other hand, if they wanted support for regular Wii games, they would have to either patch them at launch time or implement the whole thing in IOS by faking Bluetooth packets. Add the fact that IOS was apparently outsourced and... if I had to bet, I'd bet on them not adding these controls to older games.