Jesus just told a parable to the crowds that is itself an explanation for why most of them won't be able to understand his parables. There are ironies upon ironies here. But privately to his disciples, Jesus explains what's going on in plainer terms.
The rule is: "Whoever has, to him shall more be given . . . but whoever does not have, even what he has shall be taken away from him." Those who listen with the receptive heart of faith will be given yet more faith and an increasing ability to receive the truth. Those who listen with an unreceptive heart, rejecting Jesus' teaching, will become increasingly unable to find their way to the truth. The good get better, the bad get worse. So when God takes away someone's ability to hear the truth, it is a consequence of the hardness that already resides in their hearts in the first place. It is a judgment, you might say, and a sobering warning to all of us who are exposed to the Word of God on a regular basis as these Israelites were.
The function of the parable is to accelerate this process, to hasten people toward their ultimate fate, whether good or bad. For those who believe, the parable forces them to search harder for the truth, to inquire, to meditate. The truth they eventually come to understand is hard-earned and therefore well treasured. But for those who disbelieve, the parable presents only greater confusion. They dismiss the parable as foolishness and turn their backs even further on the truth. As Jesus says, "Seeing they do not see, and while hearing they do not hear nor do they understand."
The prophecy of Isaiah tells how most of Israel progressed from bad to worse. So much history and so much revelation for generations on end, and yet the trajectory of their belief did not increase to glory but declined into judgment. At the heart of their unbelief, according to Isaiah, was their unwillingness to return to the Lord and be healed. They rejected Yahweh, that's why they refused to believe. That's why their judgment is just.
But amidst every unbelieving generation there are those few who do believe. The prophets and righteous men of Israel's day were such people, and they eagerly received what little revelation was given to them. They desired more, but the time had not yet come for the truth to be fully revealed in Christ. With Jesus' coming, the disciples are the faithful few amidst their generation. Yet they are not only blessed with hearing ears and seeing eyes but possess this blessing at a time when the all the ancient promises are being fulfilled right in front of them. We already know how their lives turned out, what an abundance of faith would be given to them in the face of sufferings and persecution, and how much fruit they would someday bear in laying the foundation for the Christian church. "Whoever has, to him shall more be given."