Answer:
Because it was located in the prosperous Bosphorus Strait.
Step-by-step explanation:
The Bosphorus Strait was an important trade route during the Roman Empire and had a crucial role in the Middle Ages. Constantinople controlled these routes, turning into an important city. Beyond the economical aspects, the geography of the Bosphorus aids Constantinople to avoid attacks. The huge walls protected the city closer to the shores, and might mountains protected the city by its backs.