Because many of us may have the idea that a parable is nothing more than a story with some moral truths being portrayed, which would be wrong. We know this to be wrong, because that is not the gospel message. Jesus didn’t leave all His glory in heaven and come to earth, and live a perfect sinless life obeying the law of God, of which you and I have broken repeatedly and deserve damnation for, go to the cross in our place and on the third day rising from the grave defeating Satan, sin and death and now rules over all of creation as king of kings and lord of lords; He didn’t do all of this to make us moral, to make bad people good but rather He came to make dead people live. So we don’t believe that parables are just stories with moral truths attached to them.