in the beginning, slavery mostly
Although the people of the Byzantine Empire considered themselves Roman, the East was influenced by Greek culture, rather than the Latin of the West. People spoke Greek and wore Greek-styled clothing... The Byzantine Empire was influenced by the Hellenistic culture created by the conquests of Alexander the Great.
One of the reasons was the blocked transportation routes, another was they were giving so many materials to the army to win they put aside their other business to focus on that
