Sumo is defined as the national sport of Japan but the most popular sport is baseball.
Japanese are keen on football too but no one is dreaming to win the World Cup.
On the other hand, all Japanese are thinking just Gold medal for baseball.
Sports using balls or objects can be divided to two major categories. First one, A is a game with a limited time. Football, basketball, ice hockey and rugby belong to this category. The second one, B is a game without time limitation. Baseball, golf, tennis and volleyball belong to this category.
Then, these categories can be divided into 1) score limitation or 2) no score limitation.
A1 - nothing. A2 - Football, basketball, ice hockey, American football, handball, field hockey, rugby and etc..
B1 - Tennis, volleyball, table tennis, badminton and etc.
B2 - Baseball and golf.
B2 has no limitation, therefore it
can be any kind of upsets. Baseball cannot be finished until your team gets 27 outs from your opponent. The score is 10-0 when the top of the 9th inning finished. If your opponent scores 11 at the bottom of 9th inning, you have lost the game. On the other hand, if FC Barcelona scored 5 and Sparta Prague just 1, and there is only 3 minutes left, no one thinks about upset by Sparta Prague. May be 10-0 is impossible but 5-1 for baseball is very common for team to win after scoring 5 points at the last inning.
Then I think this system matched to Japanese who love gambling and never think of give up.
By the way, the sport which attracts the most number of audiences per game is not baseball in Japan. It is horse racing. Japanese love gambling.