The period of history that The Tale of the Heike was written in says a lot about the types of themes that we see throughout the story. While the book depicts the events of the Genpei War (1180-1185), the story existed for a long time as simply a series of oral traditions passed down and retold by the biwa hōshi, a class of traveling musicians and storytellers. The version of the story that we are reading was not written down until roughly 1370, during the rule of the Ashikaga Shogunate.
After the rule of Yoritomo, the Minamoto remained in power until 1333; however, while Yoritomo himself was an effective leader, his descendants were not nearly as strong. By the early 1300’s, the Minamoto Shoguns ruled in name only: the real power was the Hojo Clan, who ruled as regents of the Shogun. The Hojo were a distantly related successor clan to the Taira. In 1331, Emperor Go-Daigo rebelled against the Hojo. The Emperor was aided by Ashikaga Takauji, who in turn rebelled against the Emperor and set up a new government, the Ashikaga Shogunate. The Ashikaga themselves were a successor clan to the Minamoto.
The Hojo and Ashikaga conflict in the 1300’s mirrors the war between the Taira and Minamoto that we see in the book. After they came into power, the Ashikaga used their Minamoto identity to justify their government and power. The Heike story was very popular with the Ashikaga, who loved both the book and Noh theater about the story because it glorified their warrior ancestors and helped legitimize their rule. Can you think of other examples throughout history of governments making connections between themselves and past dynasties/kingdoms/great rulers in order to legitimize their rule?
Additionally, the writing of this story coincided with a transitional period of Japanese history between the Heian period and the time of the samurai. The dominant culture in Japan was changing from an art- and aristocracy-focused culture to a culture centered on warriors, bravery, and honor. As such, the popularity of great war stories with larger than life figures (ie. Yoshitsune) follows that transition. The Heike is the first true samurai story in Japanese literature.