The part of the parables that are the hardest to preach, much less understand, are the times when Jesus says that the parables are only for insiders - those with ears to hear. That can be perplexing, especially since the parables appear to be fairly straighforward and easy to understand. Furthermore, why would God exclude hearers from the goodness of the Gospel?
Adriel Sanchez explaines this part of the parables in the video, posted below.
As a side note, there are ways of interpreting the parables that may belie the "ears to hear," admonition. For instance, if you approach a parable with a "this means that, and that means this (etc.)" attitude - parsing each part of a parable to mean something that affirms the reader's current belief - then that reader is missing the point. In other words, they do not have ears to hear. They are approaching the parable from a place of stubbornness and pride.
For the most part, a parable is made to relate one aspect of the Kingdom of God to a hearer who needs the "word picture" to better understand. Contriving a parable to match self-centered presuppositions lowers the words of Jesus to that of a mere storyteller.
Instead, if you have ears to hear, then approach God's Word with reverence and humility. Let it shape your life, not the other way around. Mine the rich point that is made by Jesus in each parable, and let it change your life!