Many teach that a parable is a simple story with a heavenly meaning inferring that it helps common people understand the truth. Obviously that is not what Jesus said. Jesus clearly taught that the parables were given to hide the "secrets of the kingdom of the heavens" from those who did not need to understand. Paul told us that...
"For the present we are observing by means of a mirror, in an enigma, yet then, face to face. At present I know out of an installment, yet then I shall recognize according as I am recognized now." (1 Corinthians 13:12)
Several "secrets" were revealed to the Apostle Paul.
"To me, less than the least of all saints, was granted this grace: to bring the good news of the untraceable riches of Christ to the nations, and to enlighten all as to what is the administration of this secret, which has been concealed from the eons in God, who creates all..." (Ephesians 3:8-9)
Paul was chosen to share certain "secrets" previously "concealed". My favorite is about the change that is coming to our bodies.
"Lo! a secret to you am I telling! We all, indeed, shall not all sleep, yet we all shall be changed, in an instant, in the twinkle of an eye, at the last trump. For He will be trumpeting, and the dead will be roused incorruptible, and we shall be changed. For this corruptible must put on incorruption, and this mortal put on immortality." (1 Corinthians 15:51-53)
The vast majority of humanity have never seen a Bible, and will not hear about their salvation until AFTER they are "changed" at their resurrection. But then, all will be shown God's secrets, and realize He is not reckoning their sins against them, and that he will save them all.
"...God was in Christ, conciliating the world to Himself, not reckoning their offenses to them..." (2Corinthians 5:19)
"...and of their sins and their lawlessness should I under no circumstances still be reminded" (Hebrews 8:12)
"Here is a trustworthy saying that deserves full acceptance: Christ Jesus came into the world to save sinners...This is good, and pleases God our Savior, who wills all people to be saved and to come to a knowledge of the truth...we have put our hope in the living God, who is the Savior of all people... " (1 Timothy 1:15; 2:3; 4:10)
So why did Jesus speak in parables? Well, sometimes it is none of our business to know all the secrets of the Kingdom of heavens. But I am sure, when it becomes my business, God will let me know.