Most people know that Jesus brought forth many of his teachings by the use of parables,or stories with hidden meanings.
I have always been taught that the reason he did this was because it was easier for the uneducated common people of the day to understand what he was trying to teach by listening to stories they could relate to.
This is probably one of the answers but just the other day I heard a Rabbi speaking and he claims that the reason Jesus taught in parable form was because once a truth is brought to you,you are no longer able to ignore it or avoid it because you now have knowledge of it.
Going against what you have just learned is wrong and sinful.
So in order for a people,who were not quite mature enough in their faith,to be given a sort of break,the parables was used.
When the person was spiritually ready the truth in the parable was revealed to them,then they would be able to not only follow what they were taught but also understand and accept the consequences for not following.
To tell a person out right,who is spiritually not ready for such enlightenment,puts them in the position of committing an act of sin but not fully understanding why they have sinned,or having enough spiritual control to stop themselves.
This is probably why Jesus always ended his parables with "let those who have ears hear."
Have you ever had a time where you are either reading the Bible,saying a prayer or listening to a religious story and all of a sudden,like lightening,it becomes crystal clear,like it all makes sense.This is your parable moment and suddenly you have ears to hear and eyes to see with.
This enlightenment brings with it great responsibility it requires you to now change your thought and action process deciding if you will or will not follow what you know you should.
You now face free will with knowledge,acceptance and obedience or worldliness and sin.
Either way your eyes are wide open and your choice is made through free will.