First, when Japan came to the Tang Dynasty to select literary works, Bai Juyi was the most famous at that time, and several of them just fulfilled each other. In 838 AD, Bai Juyi was 67 years old and had been exiled back to Luoyang from other places. At that time, a Japanese envoy came to Datang to seek China literature and brought it back to Japan to study. When they met the Tang Emperor, they recommended Bai Juyi's poems. But these messengers didn't know that China literature was good at all, but Bai Juyi's poems were popular in the literary world of Datang at that time, so they paid more attention to it. Bai Juyi has been sorting out his own works. Japanese messengers are in urgent need, and Bai Juyi has ready-made ones for them.
Second, Li Bai and Du Fu have become a thing of the past, and their poems and songs have not been sorted out and circulated. In ancient times, few people sorted out their own works, almost all of which were sorted out by later generations. At that time, although Li Bai and Du Fu had passed away 100 years ago, no one had fully clarified their roles. Du Fu was never reused in the Tang Dynasty, but served as a small official guarding the armory for a period of time, so his works were not circulated. Li Bai was once famous, but it was only a flash in the pan in the Tang Dynasty, and his works were rarely selected into the famous poetry collection "Guo Xiu Ji" in the Tang Dynasty.
Third, Bai Juyi's aesthetic view is more in line with the Japanese. Japanese people have delicate feelings, and Bai Juyi was exiled many times in his life. Most of his poems are full of feelings of leisure and sadness, which are easily accepted by the Japanese and more easily accepted by the Japanese.