why are there no adults on the island?

There are no adults on the island as as the reason the boys arrive there is a plane crash, and presumably the adult passengers dd not survive. The boys may have been part of some sort of evacuation procedure. It is implied that there is some sort of war going on, as in Chapter 6 a parachutist drops from 'the battle fought at ten miles' height'.

Tracey is correct. Golding wants readers to see what kind of a society the boys will create, modeling it after what they have learned from adults. Not pretty, but very realistic.