Diocletion began to phase out the legion system well before the Byzantine Empire/Eastern Roman Empire was a standalone state. Instead of one massive, infantry-driven army, he split it into limitanei (frontier garrisoned soldiers) units and comititenses (legion-ish mobile line soldiers) units. He also put an increased focus on the use of cavalry, possibly due to the superior cavalry forces of the Sassanid empire, Hunnic forces, and various other nomadic hordes who used cavalry as the main wing of their army (The Byzantines actually more or less switched to this around the 7th-8th century).