Compared to wii, the U has additional security measures, however N goofed at several places, so fail0verflow were able to dump secret bootloaders and private keys. This allows them to run custom code in wii and *U modes.
To run homebrew in wiiU mode, large amount of work is needed for libraries and stuff, perhaps even porting Linux to it to make app porting easier. However they lost interest and nobody trustworthy in the community wants to step up and do the work (why is that sounding so familiar?). They concluded that in this age of seas of open Android and other systems (like Steambox), and locked consoles becoming ever-so-complex, it's just not feasible to do homebrew on locked systems at all any more. They don't want to help pirates so they keep the hacks and keys to themselves, except information on how to boot extra CPU cores in wii mode, which they have already published.