Do the proprietor's of those platforms want an "official" app that they are willing to put their own resources into developing? Remember, this is about Google cutting off an API key so Google isn't out any engineering/time/money resources, the "official" modifier is mostly marketing BS.
FirefoxOS doesn't need a native or official YouTube app, because the entire thing is just web pages. If Google wanted to make something official, all they would need to do is add an app manifest file with some metadata about the app.