Constantine moved his headquarters, and thereby the headquarters of the Roman Empire, to Constantinople in 324 CE. Constantine had lots of reasons to move his capital east. For one thing, he was born in modern-day Croatia. Also, he probably spoke better Greek than Latin. And plus, the eastern provinces were a lot richer than the western provinces, and from a looting perspective, you just want to be closer to where the good warring is. The enemies in the east, like the Persian Parthians and the Persian Sassanians, were real empires, not just bands of warriors. And no matter who you were in world history, if you wanted to make a name for yourself in terms of war, you really needed to be up against the Persians.