understanding blockage

Week 37  Mark  4

Jesus used many stories and illustrations to teach the people as much as they were able to understand. For instance:
• the Kingdom of God is like seed growing in a field
• the Kingdom of God is like a mustard seed
Mark says that in public the Lord only taught in parables but behind-closed-doors with the disciples he explained the meaning to them. It seems like a kind of secret society: For Members Only.
I wonder why the Lord publicly taught in riddles. Was he purposely keeping people in-the-dark (and then privately telling the secret meaning to his disciples)?
That doesn’t seem to jive with what Mark already said – that the Lord used many stories and illustrations to teach the people as much as they were able to understand. That seems like the logical thing to be doing – teaching people to help them actually learn. (Even if there was a ceiling to people’s understanding I can’t see any benefit to the Lord in lowering it.)
It makes a lot more sense to me thinking that a) the Lord was teaching people so they would learn rather than b) he was teaching people so they wouldn’t.
Of course it does look like there was a lot of uncertainty with the parables. But for me the question isn’t: why are people kept in the dark? I think as good a question  to ask is: why do people seem to prefer to avoid the light?

Note: Mark 4:33 34 (NLT). See 4:26-29 &  30-32