Why not “Istanbul”? Because the name Istanbul is really an Arabic derivative of the Greek. If you live near New York City for example and are heading into Manhattan, you don’t say “I am going to New York City.” Instead, you say “I am going to the city.”
Well, the “Istanbul” means the same thing. “Ee steen polee” is the Greek way of saying in the city (in this case the “city of Constantine” or Konstanteen-oupolee — Constantinople in English). Change the the Greek hard “p” to the Arabic hard “b” and you end up with “ee-stan-bolee” or Istanbul. The Arab speakers in Turkey know this of course. Non-Arabic speakers think the name of Constantinople actually was changed.