Why Did Jesus Say He Spoke In Parables?

When Jesus' ministry first started he often spoke in very plain language about things that everyone could understand. As His ministry progressed, he began to speak in mostly parables. This puzzled the disciples and they asked Him why. What was Jesus' response?

This was Jesus' response: "And he said to them, "To you has been given the secret of the kingdom of God, but for those outside everything is in parables, so that "they may indeed see but not perceive, and may indeed hear but not understand, lest they should turn and be forgiven."' Mark 4:11-12

Why did He respond this way? One reason Jesus taught in parables was to separate the ones who truly believed from the people who were just going along with the crowd or who were there to cause trouble. Jesus spoke in a way that those whose hearts were open and willing to hear the true meaning would, but those who were only there for a show or who thought they were holy did not really "hear" the message.