I wonder if the other streaming boxes have other channels not on Roku or Fire Box?
WatchTCM is the app available for Fire TV, Apple TV, iOS, and Android (mobile) platforms. It is not available on Android TV (at least, not mine) or Roku. I have not tried to use WatchTCM on Android (mobile) phones to cast to Chromecast, so I don't know how well that works, if at all.
TCM has chosen to partner with FilmStruck, offering TCM Select via the FilmStruck app and standalone subscription service. That's available on most platforms, including Roku, Android TV, and others that for whatever reason TCM has chosen to not provide its own app.
For my Android TV device and Roku devices, FilmStruck is the only TCM option. And, as jeffrok said, that's up to TCM as to what devices they'll provide a native app.