Hachijin Shinzo is one of eight 'unrecognized' yokozuna. The Gojo household had a protracted political struggle with the Yoshida over authority in sumo. As a result, Osaka Sumo and Kyoto Sumo respectively saw two and three of their rikishi given yokozuna licenses by the Gojo, to make for five of those eight yokozuna. Put simply, they are not recognized today because the Yoshida eventually prevailed.
Of the remaining three unrecognized yokozuna: two were given provisional licenses for context-specific usage by the Yoshida, and one was given a license by the Shinrikyo temple after an unsuccessful petitioning of the Yoshida by the Osaka Grand Sumo Association.