I have always thought he used parables to convey a message more clearly than just hard-and-fast rules.
When he gave stories that illustrated various characters in different stages of the human condition, (such as those who were prideful, ignored others who had problems, etc.), those who listened could associate their actions with one of the people.
As an example, take the man who asked "Who is my neighbor?" Jesus shows how three people, a priest, a Levite, and a Samaritan, each reacted to a man who was injured. Therefore, those listening or reading the Gospel can place themselves in the situation and see who they most closely embody in their current state, and what they can strive to become.
If Jesus had simply said, "Help anyone, regardless," there would not be such a powerful image to help someone, thus, the message would not have been conveyed effectively.