I don't think that's the reason, considering Nintendo created special JavaScript APIs for accessing the Wii U's gamepad hardware features and made them availabl

I don't think that's the reason, considering Nintendo created special JavaScript APIs for accessing the Wii U's gamepad hardware features and made them available to all sites via their browser. My bet is that they felt they needed more time to figure out how to properly sandbox it to prevent homebrew/piracy-related exploits, and that it will be released as a free installable item (or packaged with an OS update) in the next few months.