Theophylact of Ohrid

That which David had foretold in the person of Christ, I will open my mouth in parables, (Ps. 78:2.) the Lord here fulfils; as it is said, And when much people were gathered together, and were come to him out of every city, he spake by a parable. But the Lord speaks by a parable, first indeed that He may make His hearers more attentive. For men were accustomed to exercise their minds on dark sayings, and to despise what was plain; and next, that the unworthy might not receive what was spoken mystically.