In the Default rubric setting the Negative score means that each student has the full point amount for an answer and you are grading by deducting points from the total for errors.
The Positive score means that each student starts with 0 points for a question and you are building to a final score on the question by adding points for things they did that were correct.
The Negative scoring rubric is the default because that is the way most people think about grading: deducting points for errors.