As you are probably aware there had been several smaller cities in the area before Constantine (re)founded Constantinople in 324/330, like its predecessor Byzantium or Chalcedon on the other side of the Bosporus. Their founders had most likely been attracted to the location by the natural advantages you describe. But non of those cities reached the size of late antique or medieval Constantinople. One reason must have been the problem of water supply. There aren't all that many natural springs in the area where Constantine would found his city and only one fairly minor river. So drinking water had to be procured from far away hills in the west by aquaeducts. The same applies to foodstuff. A city as large as Constantinople couldn't solely rely on the agrarian production of its hinterlands. For example in Late Antiquity grain had to be imported from Egypt for Constantinople to be able to compete in size with other mediterranean centers like Rome or Alexandria.

This means that a city located at the Bosporus can only grow so much without the support of a larger state that has the ability to redistribute large quantities of drinking water and foodstuff. This state would have been the Roman Empire but before the 4th century none of its rulers felt the need to heavily invest in the enlargement of one of those settlements.

There was an independent Greek city state there named Byzantium. It was always a trading city due to its location. These states were highly independent and many did not have far reaching influence so their history is not very well covered. In Xenophon's Anabasis he has an interesting story involving the city. On his march back to Greece he had the opportunity to claim the city as his own but refused it. Of course his telling is much better than mine.