It’s entirely up to Twitch what information they want to expose to 3rd parties. There could be any number of reasons why some fields are not included in Helix, such as performance/technical reasons, lack of sufficient use cases, legal reasons, no expressed demand for such data by the 3rd party developer community, or any other reason/combination of reasons.
It works fine here, so if you’re having issues you should check your dev console and see what your error is.