Sorry, I couldn't resist. Anyway, Wikipedia tells us that the Ottomans primarily called the city Kostantiniyye during their rule and the name was only officially changed by the Atatürk government in 1930. Why the change?
Ataturk wanted to ignite Turkish nationalism and create Turkey as an entity separate from the Ottoman Empire. This involved a lot of changes that were symbolic in nature. There's a much better answer than I could ever give here.