When YouTube TV first launched it only worked on Android, iOS, and Chromecast via casting. Later they added support for casting to Apple TV, and released an app on Android TV. Now YouTube TV has added support for Roku players and TVs.
Recently (not exactly sure when) YouTube TV has added support for casting to Roku players and Roku TVs. Now with just the standard YouTube Roku Channel installed you can now cast YouTube TV from your phone or tablet to your Roku player. This opens up YouTube TV to Roku owners around the United States.
This is great news for Roku YouTube already has this so why not YouTube TV.