First, look at Luke 15:1-3,11-32 as read in church,
1 Now all the tax-collectors and sinners were coming near to listen to him. 2 And the Pharisees and the scribes were grumbling and saying, ‘This fellow welcomes sinners and eats with them.’The impression casual listeners hear is that Jesus launched right into the parable of the prodigal son, but that is not the way Luke presents it. Here are the two parables you may have missed hearing today,
3 So he told them this parable:
11 Then Jesus said, ‘There was a man who had two sons. 12 The younger of them said to his father, “Father, give me the share of the property that will belong to me.” So he divided his property between them. 13 A few days later the younger son gathered all he had and travelled to a distant country, and there he squandered his property in dissolute living. 14 When he had spent everything, a severe famine took place throughout that country, and he began to be in need. 15 So he went and hired himself out to one of the citizens of that country, who sent him to his fields to feed the pigs. 16 He would gladly have filled himself with the pods that the pigs were eating; and no one gave him anything. 17 But when he came to himself he said, “How many of my father’s hired hands have bread enough and to spare, but here I am dying of hunger! 18 I will get up and go to my father, and I will say to him, ‘Father, I have sinned against heaven and before you; 19 I am no longer worthy to be called your son; treat me like one of your hired hands.’ ” 20 So he set off and went to his father. But while he was still far off, his father saw him and was filled with compassion; he ran and put his arms around him and kissed him. 21 Then the son said to him, “Father, I have sinned against heaven and before you; I am no longer worthy to be called your son.” 22 But the father said to his slaves, “Quickly, bring out a robe—the best one—and put it on him; put a ring on his finger and sandals on his feet. 23 And get the fatted calf and kill it, and let us eat and celebrate; 24 for this son of mine was dead and is alive again; he was lost and is found!” And they began to celebrate.
25 ‘Now his elder son was in the field; and when he came and approached the house, he heard music and dancing. 26 He called one of the slaves and asked what was going on. 27 He replied, “Your brother has come, and your father has killed the fatted calf, because he has got him back safe and sound.” 28 Then he became angry and refused to go in. His father came out and began to plead with him. 29 But he answered his father, “Listen! For all these years I have been working like a slave for you, and I have never disobeyed your command; yet you have never given me even a young goat so that I might celebrate with my friends. 30 But when this son of yours came back, who has devoured your property with prostitutes, you killed the fatted calf for him!” 31 Then the father said to him, “Son, you are always with me, and all that is mine is yours. 32 But we had to celebrate and rejoice, because this brother of yours was dead and has come to life; he was lost and has been found.” ’
4 ‘Which one of you, having a hundred sheep and losing one of them, does not leave the ninety-nine in the wilderness and go after the one that is lost until he finds it? 5 When he has found it, he lays it on his shoulders and rejoices. 6 And when he comes home, he calls together his friends and neighbours, saying to them, “Rejoice with me, for I have found my sheep that was lost.” 7 Just so, I tell you, there will be more joy in heaven over one sinner who repents than over ninety-nine righteous people who need no repentance.
8 ‘Or what woman having ten silver coins, if she loses one of them, does not light a lamp, sweep the house, and search carefully until she finds it? 9 When she has found it, she calls together her friends and neighbours, saying, “Rejoice with me, for I have found the coin that I had lost.” 10 Just so, I tell you, there is joy in the presence of the angels of God over one sinner who repents.’Jesus must have told the two missing parables first for a reason. I will speculate and guess that these two stories were told first in order to prep his audience as to the identity of the father figure in the "Parable of the Prodigal Son". The first two parables point to heaven and the angels. This leads to the inescapable conclusion that the father in the third parable is God which might lead the Pharisees and scribes to assume that Jesus is making a God-like claim that he is the one who welcomes the repentant sinner.
Even if my guess is wrong, why should we second guess Jesus and cut out a large chunk of his words?
As some have commented here, the Gospel might be shortened by the Lectionary editors in the interest of time, otherwise the Sunday service might run long, in this case an additional 55 seconds. I timed it, and that is how long it takes to tell the other two parables. It would be far better to shorten the sermon by 5 minutes than to shorten the Gospel at all.
I have another theory, and that is the desire by some to avoid teaching about the supernatural in church, particularly during the sermon. By leaving out references to heaven and angels and by sticking to the earthy story of the prodigal son, the pewsitters' minds never have to leave the ground. The end result is that their imaginations are not given the opportunity to dream of loftier things.
As always, be alert for those missing verses!