There’s always a danger in trying to answer “Why” questions. But there are two thoughts the text emphasizes when it comes to Jesus’ decision to speak in parables:
1. It is a fulfillment of Old Testament prophecy (especially Psalm 78:2 and Isaiah 6:9-10).
2. The use of parables emphasizes the need for the Spirit’s help in understanding. The striking contrast between the Isaiah prophecy (Matthew 13:14-15) and the following beatitude (Matthew 13:16) emphasizes the blessedness of those who – by the power of the Spirit – are able to see the truths of the kingdom. The parables are very simple stories/comparisons – but the deep spiritual truths they convey will also resonate with hearts that have been opened to the truths of the kingdom by the Holy Spirit.