Why did Jesus speak in parables? First of all, because the things of God are beyond our human understanding, so He puts them in simple, everyday examples to help answer questions that everyone has. The question the parable of the sower answers the question “why doesn’t everyone believe?” Jesus explains the answer through this parable.
Yet the parables need to be interpreted. That’s the second reason for the parable. Not everyone is ABLE to understand the things of God. Only to those to whom God reveals the truths will understand what He is speaking about in parables. Only those who God has given the power of the Holy Spirit (all those who believe) are able to read God’s Word and understand it.
Who is this Jesus? The One through Whom God chose to reveal many of His mysteries to His chosen people.