[cafard]

Byzantium was a city for centuries before Constantine was born, and a fine place to control the grain trade from the Black Sea.

[arethuza]

There was a city there but it wasn't that large and was dramatically redeveloped after it became the new capital:

https://www.thegreatcoursesdaily.com/making-constantinople/

[danjac]

Not to mention being closer to the Silk Road trade routes through Asia.