Luke 8:10 (NKJV)
And He said, "To you it has been given to know the mysteries of the kingdom of God, but to the rest it is given in parables, that 'Seeing they may not see, And hearing they may not understand.'
You've heard it many times before when a speaker wants to further explain and clarify his message, he will say: "Let me illustrate..." Jesus purpose with parables was not so. For Jesus, parables were meant to conceal the truth from the uninitiated and to reveal deeper truths to those who had believed.
Jesus said not to "cast your pearls before swine." This means that you should not give things of great value to those who are incapable of appreciating them. He is not saying do not try to reach these people with the truth. But rather, if they do not respond to the simple truth of the gospel, they will never respond to the deep truths of God.
1 Corinthians 2:14 (NKJV)
But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned.