Turkey is a big country with its 80 million people and Ankara is the capital. But Istanbul has always been the heart of the country because of its cultural history, diversity, number of universities, employment opportunities, museums, markets etc.
Istanbul is like Newyork city of Eastern Mediterannian part of the World and it is the only city in the world located both in asia and Europe. Because of the bosphorous straight Istanbul has always been a very important and strategic city.
Istanbul has about 18 million population. It was the capital city of byzatine empire / east roman empire from 4th to 15th century, and of ottoman empire from 15th to early 20th century.