I know, Sumo wrestling is a sport of Japanese culture and is not suitable for betting. Yes, sports such as boxing today are suitable for betting, but Sumo wrestling is a sport played for a culture, not for betting.
I agree, its not entertaining and its not suitable for the betting and this sports is really a serious that people are afraid to watch, not like in boxing there is always an excitement and you can predict if who will be the win in the fight.
I think betting in sumo already exists. This article provides where you can bet for sumo sport -1xBet and Unibet.
Though unusual to have betting in this sport, yet, it is already happening.