Why isn’t Xfinity stream on Apple TV?

Developers have to program apps separately for iPhone, iPad, or Apple TV box. If Comcast creates an app for iPhone and iPad, but not for Apple TV box, then the app will not be available on Apple TV.

Request Comcast to create an Apple TV app of Xfinity Stream. Then Apple would be happy to make it available in the App Store. How to contact an app developer - Apple Support

Because Xfinity has not released the Xfinity Stream app for the Apple TV yet. You should ask Xfinity that question. Check the link below for a possible workaround.

https://robotpoweredhome.com/xfinity-stream-on-apple-tv/