Why parables?

If the way in which a man is saved is synergistic (man chooses to believe in God) – and not monergistic (God’s work alone for man’s regeneration) – then why did Jesus speak in parables, the purpose of which was to obscure the truth from those who were not His?

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