Why does Jesus speak in parables?

Mark 4:10 And when he was alone, they that were about him with the twelve asked of him the parable. Matt 13:10 And the disciples came, and said unto him, Why speakest thou unto them in parables? Luke 8:9 And his disciples asked him, saying, What might this parable be?
Mark 4:11 And he said unto them, Unto you it is given to know the mystery of the kingdom of God: but unto them that are without, all these things are done in parables: Matt 13:11 He answered and said unto them, Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given. 12 For whosoever hath, to him shall be given, and he shall have more abundance: but whosoever hath not, from him shall be taken away even that he hath. Luke 8:10a And he said, Unto you it is given to know the mysteries of the kingdom of God: but to others in parables;
Mark 4:12 That seeing they may see, and not perceive; and hearing they may hear, and not understand; lest at any time they should be converted, and their sins should be forgiven them. Matt 13:13 Therefore speak I to them in parables: because they seeing see not; and hearing they hear not, neither do they understand. Luke 8:10b that seeing they might not see, and hearing they might not understand.

Matt 13:14 And in them is fulfilled the prophecy of Esaias [1], which saith, By hearing ye shall hear, and shall not understand; and seeing ye shall see, and shall not perceive:

Matt 13:15 For this people's heart is waxed gross, and their ears are dull of hearing, and their eyes they have closed; lest at any time they should see with their eyes, and hear with their ears, and should understand with their heart, and should be converted, and I should heal them. 16 But blessed are your eyes, for they see: and your ears, for they hear. 17 For verily I say unto you, That many prophets and righteous men have desired to see those things which ye see, and have not seen them; and to hear those things which ye hear, and have not heard them.

[1]Isaiah 6:9 And he said, Go, and tell this people, Hear ye indeed, but understand not; and see ye indeed, but perceive not. 10 Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed. This is part of a speech which God tells Isaiah to deliver to the people.