Apart from other reasons, undoubtedly one of the purposes of Jesus' parables is to incite hunger in the hearts of genuine seekers after Jesus/the Truth.
If I said to my children, "Your Christmas present this year is like..." and told them a parable type story that made no sense to them, it would invoke huge curiosity and prompt many questions. It's because they are fascinated with Christmas and what they are going to receive.
For Jesus to say "the kingdom of heaven is like..." and tell a parable, it separates the interested from the non interested. Later his disciples would says "tell us the meaning." No doubt they are interested because they are his disciples, but they are also his disciples because it interests them. And of course their curiosity is rewarded with an answer.
Rev V.G.Tasker in his Matthew Commentary says "Jesus deliberately adopted the parabolic method of teaching at this particular stage in His ministry for the purpose of withholding further truth about Himself and the kingdom of heaven from the crowds, who had proved themselves to be deaf to His claims and irresponsive to His demands."1
Jesus quotes the prophet Isaiah (in 6:9) as justification for this action. "Be ever hearing, but never understanding, be ever seeing, but never perceiving."
Other commentators have suggested election as one reason, or fulfilment of scripture as another.
Or we could also consider a combination of judgement and mercy. Judgement for those who God already knows refuse Him and so truth is hidden from them. But the withholding of truth is also mercy because it will result in a lesser judgement than if they were fully aware.
It's time to ask ourselves the questions, are we interested in what God is saying to us. It is then up to us to do as the disciples did, and go to Christ saying "explain the meaning of these things to us." We are to seek wisdom and understanding, and ask for a revelation of Christ.
To the hungry is given wisdom.