Why are certain apps not on the list to put as the face on your apple watch?

The app needs to have a compatible "complication" in order for it to be displayed on a watch face,you can't just add an app to a watch face,although the app is compatible with the watch the app has to have a watch "complication" in order to work on a watch face.

The app developer is the one who would make the watch complication,maybe contact the developer and ask them what their future intensions are.