Why does Vonnegut use an objective narrator who offers no interpretation of judgements about the story he tells?

The narrative point of view is a splendid example of form contributing to meaning.  By withholding judgment, the narrator participates in the plot of the story concerning people programmed not to think for themselves.  In this way, the author behind that narrative device, Vonnegut, encourages us the readers to make judgments, to participate in shaping the meaning of the story.