Sumou is often referred to as the Japanese National Sport, however 国技 means Japanese National Art - in the sense of Art being a Skill, a Craft.
It is not considered the National Sport. If any such thing exists, in Japan it is definitely Baseball, not Sumou.
What Sumou is something much more special and precious than some national Sport.
It is taken as a treasure of Japanese history, the crystallization of the Japanese culture. And as such it is viewed as an Art. It is taken to be an Art, since it is taken to preserve some valuable aspects of the Japanese thinking.
In this aspect Sumou is one with Sumoudou, since this is what the special Art and Skill mean. The preservation of a Way that makes a man to be more than a man. It is the true embodiment of Bushidou.
The term Kokugi - Japanese National Art, was coined in the 19th century.