That's the simple specific rule that your student needs about how. "Why" is more difficult to answer second hand.
- An old-fashioned form is "Should you not call him?; "Should this show not be over by now?"
- If "should not" is being used in the sense of "if", you use, "Should you not": "Should you not wish to stay for the entertainment, you may leave straight after dinner." (But that's not a question form.)