Not everyone is able to leave the country, and if everyone leaves, will ISIS be happy with that, or will it move on to another country?
A large part of the ISIS fighting force is made up of mercenaries from other countries, such as Tunisia and Saudi Arabia. There are even tens of thousands of fighters from Europe. So, for them it doesn't matter whether the local population stays or flees.
I heard that ISIS are fighting a lot of representatives from Russia. Probably from Chechnya, but I'm not so sure. There gathered the representatives of radical Islamist groups. Terrorists have no nationality.