Jesus' answer to His disciples question seems surprising. He said that He spoke to the crowds in parables because even though His disciples were allowed to know the mysteries of the kingdom of heaven, the crowds were not.
At first this seems a little cliquish. But when you consider that anyone can become a disciple of Jesus by believing in Him then we see its not cliquish at all. In fact, by Jesus speaking to the crowds in parables He was giving them an opportunity to ask themselves the question, "What does He mean?" If they would seek to understand the meaning of what He was saying then they themselves would become seekers of the Truth.
Jesus loved these people and was trying to spark a seeking heart within them so they could do some soul searching and hopefully in the process come to realize that the answers to life's parables come directly from heaven. The human soul needs to be enlightened with the wisdom from above in order to really understand life's truths.
Jesus spoke to the crowds in parables, playing the part of mediator between God and man that He was called to do, even before He died on the cross for the sin of mankind. What great love He gave--to live and die even for those who didn't, and would never, believe.