Answer:
Because of the position of the city within the surrounding trade routes and the strategic importance of such a city.
Step-by-step explanation:
Emperor Constantine saw the amazing strategic and trade options that such a city would offer. It was on the European side of the Bosporus strait and that allowed it to be well defended and to control what moved in those waters and at what price. Emperor Constantine made it the new capital of the empire in 324 CE.