Answer: for many people to not understand (Matthew 13:10-17, Mark 4:10-12, Matthew 13:35).
The following, listed scriptures are not actual parables (but are referring to parables).
10 And the disciples came and said to Him, “Why do You speak to them in parables?”
11 He answered and said to them, “Because it has been given to you to know the mysteries of the kingdom of heaven, but to them it has not been given. 12 For whoever has, to him more will be given, and he will have abundance; but whoever does not have, even what he has will be taken away from him. 13 Therefore I speak to them in parables, because seeing they do not see, and hearing they do not hear, nor do they understand. 14 And in them the prophecy of Isaiah is fulfilled, which says:
Hearing you will hear and shall not understand,
And seeing you will see and not perceive;
15 For the hearts of this people have grown dull.
Their ears are hard of hearing,
And their eyes they have closed,
Lest they should see with their eyes and hear with their ears,
Lest they should understand with their hearts and turn,
So that I should heal them.’
16 But blessed are your eyes for they see, and your ears for they hear; 17 for assuredly, I say to you that many prophets and righteous men desired to see what you see, and did not see it, and to hear what you hear, and did not hear it.
10 But when He was alone, those around Him with the twelve asked Him about the parable. 11 And He said to them, “To you it has been given to know the mystery of the kingdom of God; but to those who are outside, all things come in parables, 12 so that
‘ Seeing they may see and not perceive,
And hearing they may hear and not understand;
Lest they should turn,
And their sins be forgiven them.’”
35 that it might be fulfilled which was spoken by the prophet, saying:
“ I will open My mouth in parables;
I will utter things kept secret from the foundation of the world.”
If you think about it, most people that have ever lived have never read the Bible or have even heard of Jesus Christ. God is simply not calling everyone now. He is more than powerful enough to get the gospel to everyone — if He chose to. He’s allowing most people to live the way they feel is right, but God will give everyone a chance to change their ways and repent. Everyone that has every lived will have their chance for salvation.