This is an interesting question requiring us to engage in some conjecture. We can only surmise about what the reasons for this might be.
Some evidence we might consider: Jewel and Addie are clearly delineated from the rest of the family. Darl recognizes this. (Perhaps, in part, Darl's recognition and articulation of Jewel's "otherness" serves to offer two reasons for Jewel's sole chapter. 1) Darl speaks for Jewel, to some extent. 2) Jewel is not fully part of the family. His mother is "a horse".)
Jewel is Addie's favorite, sired in her lone fit of passion and love and wildness. Jewel is her rebellion and her, well, her jewel. As a product of these emotions, Jewel is not truly "kin" to Darl, Dewey Dell, Vardaman and Cash.
Like Addie, he receives only one chapter. Like Addie, Jewel is defined by his bitterness.
When he does talk, he usually curses, exhibiting a persistent rage.
This pair of characters forms a quiet set. They keep their own counsel, as it were. Offering more articulation to these characters in the novel might change the dynamic that renders them as set apart and silent. We might wonder if the quietness of these two characters is meant to express their shared sense of repression and deep compromise.
In order for each of them to live they must sneak off at night. They must transgress the family boundaries. Inside the family and inside the novel they can find little that satisfies or expresses them beyond mere gesture.