Why did Jesus keep some in the dark by using parables?

I once heard someone say that parables are the best way to teach, because Jesus used them.  Of course this makes no sense because parables are very difficult to understand unless explained. Jesus said this when the disciples asked why he was speaking in riddles through so many metaphors: >The disciples came to him and asked, “Why do you speak to the people in parables?” 11 He replied, “Because the knowledge of the secrets of the kingdom of heaven has been given to you, but not to them. 12 Whoever has will be given more, and they will have an abundance. Whoever does not have, even what they have will be taken from them. 13 This is why I speak to them in parables: “Though seeing, they do not see; though hearing, they do not hear or understand. (Matthew 13:10-13) So the question is why would Jesus purposefully keep much of his teaching secret through parables? Why would he want some not to see and not to hear?