I’d always assumed it was because Confucianism more suited the philosophy and needs of military strongmen, but apparently the reasons were even more directly military:
“‘The Tendai establishment on Mt Hiei was razed to ashes by Nobunaga in 1571 because of its participation in an unsuccessful military alliance against the strongman… After Nobunaga was assassinated in 1582, his ally Hideyoshi took charge of the campaign to unify Japan… In 1584, an army of fifteen thousand Shingon troops unwisely attacked Hideyoshi’s stronghold in Osaka”
Buddhism: A History by Noble Ross Reat pg 213
