It's interesting that Jesus often ends the telling of a parable with the phrase "He who has ears to hear, let him hear." (Matthew 11:15, 13:9, 43; Mark 4:9, 23; Luke 8:8) During Jesus' life and ministry, much of Jesus' teaching was, indeed, veiled, but only to those who did not have ears to hear. Perhaps those who really wanted to follow Jesus would either contemplate the parables until they understood them, or as happened to Peter, God would reveal the meaning of the parables to those who had ears to hear. The scoffers would not be force fed with the teachings of Jesus, but could cast them away. As C.S. Lewis noted, God cannot ravage--He can only woo.
The Meaning of Parables was veiled only for a short time
It is really important to note, however, that this veiling only occurred during the three years of Jesus' ministry. Immediately after the resurrection, the proclamation of Jesus as the risen Lord went out to all people, not veiled in parables, but in clear teaching. In fact, the more difficult parables that were explained in private to the disciples are now accessible to all people. In fact, the disciples who freely recorded the meanings in the gospels also likely publicly proclaimed the meaning to all people then as well.
Many for whom the parables had been veiled likely came to faith in Jesus later
On the day of Pentecost, Peter preached to the thousands of Jews who had gathered for the feast. It is quite reasonable to assert that many of these had been among those that had heard the parables of Jesus and had not understood them, and then rejected Jesus and called for His execution. Peter lays this charge against them as he speaks. However, we are informed that 3,000 of these people became followers of Jesus on that day alone.
So, the fullness of the Gospel was veiled for a short time. Some people's hearts were hardened, and Jesus did not overwhelm them to the point of thwarting their own free will to reject them. Others were perhaps not ready at that time, but did accept Jesus later. Whatever the reason, the veiling was for a very short time, as today there is no "secret knowledge" in Christianity. Anyone can read the parables and the explanations given to Jesus' disciples in private. Indeed, the reader is invited into the most exclusive moments in the life of Jesus.