The boys on the plane were being transported to a "safe place" due to atomic bomb threats during WWII. We are not told the plane is attacked, but it has crashed, and the adults traveling with the boys are dead.
Lord of the Flies
the boys have been traumatized by the attack. the fact that all the adults on the aircraft have been killed, adds to their anxiety - they are unsafe, vulnerable and insecure. they have just witnessed the destruction of something which was meant to provide them safety and bring them to a safe haven.
the island would provide them safe refuge and stability. it would be a place where they canset up base, organize, think of a way to get out and stay until help arrives.