Why was Constantinople called the "New Rome"?
The 1,000 year old city of Byzantium was repurposed in 326 AD as a new capital for the Roman Empire by Constantine the Great, hence its new name.
In 324 AD Constantine the Great had reunited the East and West halves of the Roman Empire again, and repurposed the city of Byzantium as his new capital. The city had been selected as it was recently rebuilt along modern lines after having been destroyed a century earlier, but would also stand as a symbol of the growing power of the Eastern Roman Empire.
As the new capital, even more money funded new construction in the city and Constantinople soon became the most populated and wealthiest city in what had been the Roman Empire for many centuries.