Why did islam spread so quickly in the 600?

Islam spread quickly in its religion. Many pilgrims originated from the Islamic Empire and trade routes were everywhere. This caused the pilgrims to travel within these routes in order to preach about Islam, therefore leading to the result of Islam spreading and causing believers to rise.

It wasn't really Islam that was spreading. It was the Muslims and in their wake, Islam. It was simple enrichment in their own people. 

Iraq because its lack of policing and government concern