Matthew 13:35, “This was to fulfill what was spoken by the prophet: “I will open my mouth in parables; I will utter what has been hidden since the foundation of the world.”
“Jesus spoke in parables to explain spiritual truths, but those who had already rejected Jesus did not have divinely enlightened minds with which to perceive these truths, and no amount of explanation would make them understand (1 Cor. 2:14). They could watch and hear Jesus with their physical eyes and ears, but they were not capable of understanding the truth in their hearts because they had rejected him (2 Cor. 4:3, 4). Those who accept the true light they have been given will receive even more light, while those who turn away from the light continue to be increasingly shrouded in darkness (v. 12). The word “for” at the beginning of verse 15 should be understood as having the same meaning as “because.” People do not hear and see because their hearts are full of wickedness; consequently, they fail to understand the truth that has been given them. They are so opposed to God’s message that they harden themselves against it, lest they should understand it and ask forgiveness of God. Once they reject Jesus, they also reject the possibility of understanding the parables that Jesus told (Isa. 55:6-8).” ESV Hebrew-Greek Key Word Study Bible
Speaking in parables was a fulfillment of prophesy.
Psalms 78:2, “I will open my mouth in a parable; I will utter dark sayings from of old.”