Can you explain why it is supposedly so difficult to develop games on the Wii U?

[fleischr]

With specific regard to Elliott Quest, it was built primarily with HTML5, CSS, and JavaScript.

Porting it to the WiiU requires knowledge of the Nintendo Web Framework which interfaces with that. Basically, it's a HTML5-JavaScript framework you have to learn -- not unlike Angular, jQuery, Cordova or others. It's not necessarily difficult - it's just you need to take the effort to learn it much like anything else.