Jesus speaks to people in parables:
The parables are for those whom have been given the ability to know the mysteries of the kingdom of heaven.
To those who know the mysteries more will be given.
Those that don't have more will be taken away.
People's hearts have grown dull
They have closed their eyes they don't listen with their ears.
If they did see with their eyes and hear with their ears and understood with their hearts Jesus would heal them.
Many prophets and righteous men longed to see what His disciples were seeing in person. They were blessed because they got to see Jesus first hand.
I think it comes down to the heart. If the heart doesn't want to learn or desire to understand, then it's not going to see or hear the truth either. It's like I could tell my child 10 times to do something, and if their heart isn't in it and chooses to ignore they aren't going to see or hear to do it.
That's what those that came to Jesus with a heart against them. They weren't going to hear the truth or see the signs because their hearts didn't want to.
There is a few verses on the heart:
Guard your heart with all diligence for from it flows the springs of life.
The heart is more deceitful than all else and is desperately sick who can understand it?
Our heart's our sick. We need to guard them. When we turn our hearts toward learning the truth and seeing Jesus He gives more to our hearts. But when we turn away from the truth we get further and further away.
We need to be careful to listen and see. And keep our hearts turned toward Jesus.
Lord please guard my heart.