This is why I speak to them in parables, because seeing they do not see, and hearing they do not hear, nor do they understand. Indeed, in their case the prophecy of Isaiah is fulfilled that says:
“‘“You will indeed hear but never understand,
and you will indeed see but never perceive.”
For this people’s heart has grown dull,
and with their ears they can barely hear,
and their eyes they have closed,
lest they should see with their eyes
and hear with their ears
and understand with their heart
and turn, and I would heal them.’
But blessed are your eyes, for they see, and your ears, for they hear. For truly, I say to you, many prophets and righteous people longed to see what you see, and did not see it, and to hear what you hear, and did not hear it.
Matthew 13:13-17 ESV
He used parables so that the hearts of those rejecting would not be hardened further. The same sun that softens the wax hardens the clay; and so the very same gospel message which breaks down honest hearts and leads to repentance, hardens the hearts of the dishonest and confirms them in their path of disobedience.