Because the Turks see the Hagia Sophia as an important artifact. Also, a tradition from the Ottoman times states that Islam is the fulfiller of Judaism and Christianity, and they are all Abrahamic and that no division should take place between the three. So, the Turkish government has taken this duty into their hands and are now preserving the Hagia Sophia for the younger generations of the world to see how the Turkish government and holy Muslims want to keep peace between the religions of the world, and not only that, but how the Turkish government preserved these artifacts to show the later generations of the world intact history.
