Istanbul is not the capital of Turkey because the capital of Turkey is Ankara. Istanbul is Turkey's largest city, known for its historic landmarks and cultural attractions, but it is not the capital.

Ankara, located in central Turkey, was chosen as the capital of the Republic of Turkey when it was founded in 1923, following the collapse of the Ottoman Empire. At the time, Ankara was a small and relatively insignificant town, but it was chosen as the capital because of its strategic location and its distance from the borders of the new state.

On the other hand, Istanbul was the Ottoman Empire's capital for nearly 500 years and is the country's largest and most populous city. It is known for its rich history and cultural heritage and is a significant economic, cultural, and tourist hub.

Despite its status as Turkey's largest city, Istanbul is not the capital because the government, parliament, and other official institutions are located in Ankara. Istanbul plays a significant role in Turkey's political and economic life, but it is not the official seat of government.