Why did ISIS declare the city of Raqqa in Syria as its capital?

Raqqa is the capital of ISIS. It is located in Syria along the Euphrates River. During the time in 2013 when the Syrian government, Islamic State of Iraq and the Levant and the Islamic State of Syria were fighting each other in order to either defend or capture this city. Due to its location on the Euphrates River, this would be a desirable city because of transportation reasons being that close to the river. However, in June of 2017, the United States and other forces that are fighting terrorism, they fought to take Raqqa away from ISIS.

After much fighting, the Syrian Democratic Forces were able to gain back the city. This battle was known as the Battle of Raqqa.