Why did Constantinople flourish in the 5th century, while Rome fell into decay? What facts can confirm this?

The main fact is the construction of the great wall by Theodosius II around Constantinople; completed in 450, has survived to this day, now its Turkish name is Rumeli-Gissar (Romei fortress). Constantinople was a trade crossroads from Europe to Asia, from the Mediterranean to the Black Sea, etc. Rome flourished while it was the center of a great power, and after the collapse of the Empire it lost its significance as a trading capital.