i think the city had several names and the official byzantine name was after constantine, it makes sense that after the ottomans took the city they didn’t keep honoring the dude they took it from. they started using other names along with constantine’s, and and eventually this one won out.
Official name of Constantinople was Konstantiniye in Ottoman, just pronounciation of same word.
And Actually Istanbul is not named by Turks, it is not Turkish. Most believed theory is that when city grew in 18th century, like circles around old city by suburs and outskirts, Greeks among new residents started tlaks about these areas as stampoli (or however it is written in Greek), translate as “stam” means “to” or “near”, “poli” means “city”. And after a while, since population and area of old city became so small in comparison, by 19th century locals from all nation started called city Stampoli or however they can pronounce, such Turks as Istampul. That became İstanbul (easier to say) and it wasn’t until Turkey it was official name.
