Rome remained the sole capital of the empire until Emperor Constantine I (ruled 312-337), for strategic reasons, made the city of Byzantium on the Bosporus into a second, new capital, thus shifting the imperial centre of gravity to the east. Constantine I took this step for two reasons. First, increasing military pressure from the Goths and Sarmatians on the Danube and the Sassanians on the Euphrates called for the presence of the emperor and the organs of government close to the threatened eastern border. Second, the new capital controlled the maritime trade with Egypt and the Middle East, as well as the continental trade routes that linked Europe with Asia.Baumer, The Church of the East, p. 11
Better profit and fear of the barbarians, plus a leader not afraid of upsetting the traditionalists. Remember, this is the same Constantine who also called for the council of Nicaea and made Christianity legal and then later the official religion of the empire. The man was obviously OK with shaking things up.