Because it is New Rome

Constantinople was not an apostolic see. It had no founder among the Twelve, no martyr's grave, and a bishop who in 325 answered to Heraclea. Canon 3 of the council of 381 gave it the primacy of honour after Rome anyway, and gave the reason in the text: because it is New Rome. Precedence followed the empire rather than the apostles, and Rome spent the next seven hundred years declining to accept the principle.

The Seven Ecumenical Councils (NPNF series 2 vol. 14) · NPNF2 XIV — Constantinople I canon 3