Because the friendship was based entirely on truth and nothing else. That means that even though Jonathan and David should have been natural enemies because David could not take over the monarchy if Jonathan laid claim to it since King Saul was Jonathan’s father. Nevertheless, they recognized the depth and truth of their friendship and they did not let anything get in between of it. When Jonathan realized that his father's hatred for David was irrational and based on falsehood he would not be a party to it.