Lord of the Flies by William Golding is set on an island. All of the characters in the novel are on the island because of a plane crash, and all of them are British boarding-school boys. While all of the boys survived the plane crash, none of the adults did, and there is no real reason why.
One of the oddest aspects of this otherwise effective work is exactly what happened when the plane crashed. We do know that there is a "scar" cutting through the trees where the plane cut a path. This exchange between Piggy and Ralph gives us a few details:
He looked up and down the scar.
“And this is what the cabin done.”
The fair boy reached out and touched the jagged end of a trunk. For a
moment he looked interested.
“What happened to it?” he asked. “Where’s it got to now?”
“That storm dragged it out to sea.
We also know that the boys were, at least according to Ralph and Piggy, somehow "dropped" off before the plane left the island and sank in the ocean.
“He must have ﬂown off after he dropped us. He couldn’t land here. Not in a place with wheels.”
“We was attacked!”
“He’ll be back all right.”
The fat boy shook his head.
“When we was coming down I looked through one of them windows. I saw the other part of the plane. There were ﬂames coming out of it.”
Finally, we know there was a "man with [a] megaphone" in the "passenger cabin" and a pilot who was "up front."
Now, how every boy managed to get dropped off from a plane that was somehow on fire and moving fast enough to cut a path through trees is unclear. How all of the boys not only lived but were completely unharmed by the crash and yet the adults did not survive is also unclear. So the answer to your question has to be that we do not know and the answer is not knowable. The more important thing, however, is that it must not matter.
This is a symbolic novel, and because of the point he wanted to make, Golding somehow had to get a group of proper English schoolboys on an island without adults; how they got there matters much less than the fact that they are there. If the details of the crash mattered, Golding would have given them to us. These gaps in information are not accidents but a kind of request by the author, asking his readers to suspend their disbelief and just watch (read) what happens to the boys who did survive.
Golding's purpose in writing, he says, is to make the point that humans must choose right instead of wrong because human nature will always tempt us to do the opposite. Personal choice and self-control must be coupled with the restraints imposed by society; without these things, society will devolve into savagery. To make this point, he could not have any authority figure who would help the boys maintain their civilized behavior.
In short, none of the adults could survive the crash, so Golding made sure they did not.