Why did the Byzantine Empire decline after the death of Justinian?

Because Justinian pursued an illiterate financial policy: unbearable taxes and maintenance in the luxury of his court. Due to the extravagance of the emperor and his entourage, the treasury was very quickly emptied and by the time of Justinian’s death it was practically ruined. At the same time, after his death, raids on Byzantium by the barbarians became more frequent, and his heir Justin was mentally ill and could not fully govern the state.