Constantinople was the capital of the Roman Empire, after the capital had been moved from Rome.
Constantine lied on the shore (Rome didn't) - which meant that it could be a port, and ports are very good for trade. Not only this - it had access to both the Mediterranean and the Black sea- this meant a lot more trade opportunities.
Further, it had access to Europe and to Asia via land routes: all the merchants from those continents wishing to go to the other continent would need to come to Constantinople if they wanted to use the shortest route!
So it was a cross-roads for many trade routes.
