Why was Constantinople the capital of the eastern roman empire

The Straits of Marmara connect the Black Sea with the Mediterranean Sea and separate Europe from Asia. They lie roughly in a SW to NE direction. The straits are narrow and one particular site on the European coast was settled as an ancient Greek colony called Byzantium. It was an excellent choice for a port, with a deep and easily defended harbor.
After Constantine I won his civil war to become Emperor, he knew that Rome was a decaying city. The cities of the East were far richer and partly in order to be closer to this richer source of revenue he built a new splendid capital named after himself, Constantinople.
But it wouldn’t have thrived if it hadn’t meant something to the people. After Constantine I declared official state toleration for Christianity (declaring it the only state religion came later), Constantinople became the premiere Christian city, and that caused a tremendous boom in church spending on new cathedrals, etc. Also due to its excellent harbor and skilfully built defensive walls the city was impregnable until cannon finally brought it down in 1453.

It was located in an easily defendable location and was situated on many important trade routes.