The Notitia Dignitatum (late 4th to early 5th century) already calls the city Constantinople. Is it because Procopius received a more Greek education that he uses the older name?
Procopius uses the word because it's a classicism. Essentially, classicizing writing in Byzantium makes an attempt to use a high Attic style and when writing historiography, to use terms that would appear in ancient Greek histories, mainly Thucydides but also Herodotos and Xenophon. It's one way for Procopius to signal his education to his readers and to make it clear to them that he was writing history in a classical mode (Averil Cameron, Procopius and the Sixth Century, 20, 24-5). That said, Procopius is not perfectly consistent. For example, he calls a particular group from beyond the Black Sea Massagetai, but then also claims that contemporaries used the term Hun (Wars 3.11.9, 11).