Why is The Chrysalids written in a first-person narrative?
Let us consider the aspects of first-person point of view. Because of the intensely personal nature of the narrative, because after all we see everything through the eyes of one central character, we as readers become very emotionally involved in the struggles and challenges of this one character, as we see everything from their point of view. This is very important in this excellent dystopian novel, because, through our access to David's thoughts and feelings, we are able to appreciate his danger and the way that his gift places him in such a perilous position. It also helps us side against the barbarity of David's father and the society at large, which would callously mutilate and exile children such as David and Rosalind.
Of course, we cannot escape that any first-person narrative is essentially limited and partial in its nature. This of course helps raise the suspense for us as readers. An omniscient point of view would give access to the thoughts and actions of all characters. By only being able to see things from David's point of view, we, like he is, are left in suspense as we wonder when and if his secret will be discovered and what will happen to him.
Lastly, I think the first-person narrative is essential as well to help us understand the gift that David and the others have. It is an intensely inter-personal gift that would be difficult to describe from a third person perspective. Having it described from David's perspective helps us as readers to appreciate how it creates a profound link between the group.