The disciples asked that question directly of Jesus (Matthew 13:10). He answered that God gave the disciples spiritual understanding and that no one is able to comprehend the truth without this gift. "Therefore I speak to them in parables," He said, adding, "because seeing they do not see, and hearing they do not hear, nor do they understand" (verse 13).
That is, the parables did not make the meaning any clearer for people without this gift of spiritual insight. But the parables did not obscure their comprehension. Rather, God did not give them the ability to know the truth at that time. Because Christ chose easily recognizable illustrations and objects from everyday living for His parables, they actually helped to illustrate the truth to those God called to conversion.
For more insight, please read our booklet Jesus Christ: The Real Story.