Because Hudson has one. Deca Sports 3D, like all Deca Sports games, have ten sports on one disc or in this case cartridge. Tennis, soccer, "trampoline", and sumo wresting are in the Nintendo 3DS game.
Back in the Super Nintendo’s heyday Tecmo had a long running sumo wresting game series. These days the sumo scene hasn’t received as much attention from video game publishers.
3D sumo wrestling makes its debut in spring 2011 when Deca Sports 3D comes out in Japan.