Jesus explains why and in so doing he cites some passages that give even more elaboration. The main reason is to prevent people from repenting and being forgiven or healed.
Here are some proof passages:
And when he was alone, those around him together with the twelve began
asking him about the parables. And he said to them, “To you has been
granted the secret of the kingdom of God, but to those who are outside
everything is in parables, so that ‘they may look closely and not
perceive, and they may listen carefully and not understand, lest they
turn and it be forgiven them.’ ”
Note that the passage being cited is from Isaiah, which provides more details:
And he said, “Go and say to this people, ‘Keep on listening and do not
comprehend! And keep on looking and do not understand!’ Make the heart
of this people insensitive, and make its ears unresponsive, and shut
its eyes so that it may not look with its eyes and listen with its
ears and comprehend with its mind and turn back, and it may be healed
Then I said, “Until when, Lord?” And he said, “Until the
cities lie wasted without inhabitant, and houses without people, and
the land is ruined and a waste, and Yahweh sends the people far away,
and the abandonment is great in the midst of the land. And even if
only a tenth part remain, again she will be destroyed like a terebinth
or like an oak, which although felled, a tree stump remains in them.
The seed of holiness will be her tree stump.”
So the idea is that God will prevent the nation (of Israel) from hearing until it is wasted and only a small remnant remains. Note the seed of holiness is a reference to Messiah which will start growing in that wasted/abandoned land.
As another proof text, look at 1 Cor 2.6-14:
Now we do speak wisdom among the mature, but wisdom not of this age or
of the rulers of this age, who are perishing, but we speak the hidden
wisdom of God in a mystery, which God predestined before the ages for
our glory, which none of the rulers of this age knew. For if they had
known it, they would not have crucified the Lord of glory. But just as
it is written, “Things which eye has not seen and ear has not heard,
and have not entered into the heart of man, all that God has prepared
for those who love him.” For to us God has revealed them through the
Spirit. For the Spirit searches all things, even the depths of God.
For who among men knows the things of a man, except the spirit of the
man that is in him? Thus also no one knows the things of God except
the Spirit of God. Now we have received not the spirit of the world,
but the Spirit who is from God, in order that we may know the things
freely given to us by God, things which we also speak, not in words
taught by human wisdom, but in words taught by the Spirit, explaining
spiritual things to spiritual people. But the natural man does not
accept the things of the Spirit of God, for they are foolishness to
him, and he is not able to understand them, because they are
There, some more details are revealed:
- If the world could see Christ, they would not have crucified him
- The natural mind is an enemy of God and cannot see him, and even considers the things of God to be foolish.
- The only way someone can see the things of God is if the Spirit of God reveals it to them. Thus Jesus can only be seen by his disciples.
If you want to know why is God doing this, or what the plan is for humanity as a whole, there are different interpretations based on your theological tradition
Personal interpretation: The land is a reference to you, and so you will not hear/see until you are ready and those things you trusted in are destroyed
Dispensational intepretation: God is blinding Israel to make room for the ingathering of gentiles
Calvinist interpretation: Only a small remnant of "the elect" may be saved and everyone else is blinded and prevented from repenting
Other interpretations are also possible. I'm not going to get into those debates here, you can find lots of resources on the web, but the scriptures are clear that God is responsible for hardening hearts and preventing people from seeing just as God is responsible for revealing Christ to those he has chosen.