Because it was a Roman city and more importantly, it was a capital of Eastern roman empire.
Explanation:
Because of Constantinople's strategic position in the Asia minor it has changed hands many times with the different powers of the area.
Broadly speaking it has been around for as before as the early roman republic in its current form wheres it got its enormous strategic influence.
When the empire was demarcated into two it became one of the capitals of the empire. It was the capital of the eastern wing of the empire while Rome remained the capital at the West.
