The short answer is that in the modern era, after 1923, the new country of Turkey wanted to use only the name Istanbul. More information from other contributors is here:
other responses:
The name Istanbul is derived from the Greek word istimbolin which means in/to the city and was used to indicate Constantinople, which was often just called The City. It was already in use by the Turks in common speech before they conquered Constantinople in 1453. However, during the period of the Ottoman Empire two names seemed to be in usage until the 19th century. The name Kostantiniyye appeared as the place of provenance of documents of the court and the chancellery and on coins up to the 17th century and reappeared on coins in the 19th century. The name Istanbul was associated with the highest magistrate (Istanbul efendis) and the highest military commander (Istanbul agasi).
With the establishment of the Turkish Republic in 1923 Istanbul became the sole name of the city. With the Turkish Postal Service Law of 1930 Istanbul became the official name and foreigners were requested to adopt this name in their languages and drop other names. In Europe the use of the name Constantinople had remained widespread. The Turkish postal services would no longer deliver mail sent to Constantinople instead of Istanbul.
Istanbul
It has changed name twice. Byzantium - Constantinople - Istanbul.
Constantinople was conquered by the ottoman Turks in 1453. The Turks used both the name Kostantiniyye in their official documents, and Istanbul was the was the popular name. The Europeans continued to call it Constantinople. In 1923 the Turkish government decided that Istanbul was to be the sole name of the city.
the old name of istanbul was constantinople
It was still called Constantinople by some, but The Turks called It Istanbul. The name is derived from Greek: εις την Πόλιν which means in the city. The Turks hear it as Istanbul and the name stuck.
it was changed to istanbul. This is wrong^
Byzantium changed to Constantinople to Istanbul.
Istanbul
Isanbul was called Constantinople in the Byzantine times.
It was renamed Istanbul (meaning 'to the city') in 1930.
No one damaged Constantinople to Istanbul.As it was conquered and overrun the new rulers changed the name.
it was originally named Constantinople after king constantinople. When it was taken they changed the name.
Istanbul
The former name of Istanbul is: Constantinople Before is was renamed Constantinople (in honor of the Roman emperor Constantine, this city was called Byzantium and New Rome).
Istanbul was Constantinople now Istanbul Constantinople
It has changed name twice. Byzantium - Constantinople - Istanbul.
Constantinople was conquered by the ottoman Turks in 1453. The Turks used both the name Kostantiniyye in their official documents, and Istanbul was the was the popular name. The Europeans continued to call it Constantinople. In 1923 the Turkish government decided that Istanbul was to be the sole name of the city.