Reportedly, it is because "Iyayotsugi," which was a historical tale written by FUJIWARA no Takanobu (a son of Jakucho before Jakucho became a priest) and does not exist any more, treated the period in question.
藤原隆信（寂超在俗の子）の著である歴史物語『弥世継』（いやよつぎ、現存しない）がその時代を扱っていたためとされる。