To answer the question, we must first determine what genre the gospel of John actually is. Unlike the synoptic gospels, John doesn't seem overly concerned with chronology. It doesn't seem be a Greek-style biography or history. Instead, commentators often speculate John to be a series of discourses or a thematically-arraigned work compiled over many years. For instance, Jesus drove the money changers and sacrificial-offering vendors out of the temple at the start of his ministry rather than the end as in the Synoptic tradition. John's account seems unlikely as this act must have been a major contributing factor to Jesus' crucifixion and would be hard for the Jewish religious leaders to overlook for several years. But cleaning up corrupt Jewish practice does fit in well thematically for John as it helps establish the claim that "grace and truth came through Jesus Christ." (John 1:17 ESV)
Since John is willing to juggle the timeline in order to build upon a theme, we should look at the gospel's account of the Last Supper to see why this material wasn't placed there. After washing his disciples's feet, Jesus turns to the matter of his betrayal in John 13:18-27 (ESV):
I am not speaking of all of you; I know whom I have chosen. But the Scripture will be fulfilled, ‘He who ate my bread has lifted his heel against me.’ I am telling you this now, before it takes place, that when it does take place you may believe that I am he. Truly, truly, I say to you, whoever receives the one I send receives me, and whoever receives me receives the one who sent me.”
After saying these things, Jesus was troubled in his spirit, and testified, “Truly, truly, I say to you, one of you will betray me.” The disciples looked at one another, uncertain of whom he spoke. One of his disciples, whom Jesus loved, was reclining at table at Jesus' side, so Simon Peter motioned to him to ask Jesus of whom he was speaking. So that disciple, leaning back against Jesus, said to him, “Lord, who is it?” Jesus answered, “It is he to whom I will give this morsel of bread when I have dipped it.” So when he had dipped the morsel, he gave it to Judas, the son of Simon Iscariot. Then after he had taken the morsel, Satan entered into him. Jesus said to him, “What you are going to do, do quickly.”
All three Synoptics mention the prediction that someone at the meal will become a betrayer, but none of them quote Psalm 41, which makes specific mention of bread. Mark agrees with John that the betrayer will dip bread, Matthew rephrases it to be dipping his hand, and Luke says the hand of the betrayer is at the table without referencing dipping at all.1 It's possible that both Luke and Matthew deemphasized the bread's connection to Judas to avoid clouding the picture when Jesus introduced the sacramental usage of bread. John seems intent on bringing in the prophesy callback to Psalm 41 and so he needed to put bread in Judas' hand. His solution to the confusion over bread is to move the sacramental meaning to much earlier in the story.
Going back to John 6, the context is the miraculous feeding of 5,000 men. As in Mark and Matthew (but not Luke), Jesus immediately orders his disciples to cross the Sea of Galilee and then follows them by walking on the water. Unlike the other gospels, John picks up the theme of the Jews looking for a messianic king. Apparently, the people who had been fed realized how very useful such power would be in military situations and were prepared to set Jesus up as king. As usual in John, Jesus is insistent that his work on earth isn't primarily to solve physical or practical problems2, but to solve spiritual problems.
At this point, John records one of Jesus' I am speeches. According to Wikipedia the are:
- "the bread of life"[6:35]
- "the light of the world"[8:12]
- "the gate of the sheep"[10:7]
- "the good shepherd"[10:11]
- "the resurrection and the life"[11:25]
- "the way, the truth, and the life"[14:6] and
- "the real vine"[15:1]
Each of these speeches develop a different aspect of Jesus' divine claims and are therefore important themes of the Gospel. When Jesus claims to be "the bread of life", John finds an ideal time to insert the material concerning eating his flesh and drinking his blood. Interestingly, John does hint at the original context of the words. John 6:64 (ESV):
But there are some of you who do not believe.” (For Jesus knew from the beginning who those were who did not believe, and who it was who would betray him.)
Now many commentators feel John was among the very last New Testament books produced and it was certainly written after the Sacrament of the bread and wine was being practiced regularly by the church. Certainly, Paul wrote as if the practice were firmly established in 1 Corinthians 11:23-26 (ESV)
For I received from the Lord what I also delivered to you, that the Lord Jesus on the night when he was betrayed took bread, and when he had given thanks, he broke it, and said, “This is my body which is for you. Do this in remembrance of me.” In the same way also he took the cup, after supper, saying, “This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.” For as often as you eat this bread and drink the cup, you proclaim the Lord's death until he comes.
So I'd suggest that John is throwing a different light on the already established practice of remembering Jesus via bread and wine rather than instituting or giving the historical justification for the sacrament.
John mentions bread one other time: during his resurrection appearance to the fishermen. Jesus prepares fish and bread to feed his disciples for breakfast. After the meal, he extorts Peter three times to feed his sheep3. It's hard to dismiss the idea that Jesus is referring back to both the "bread of life" speech and to the prophetic reveling of his betrayer. Here Jesus helps Peter get off the hook for his previous denial of Christ and to point him in the right direction as he takes over the leadership of the church.
As always, the different emphasis of each gospel corresponds to the authors' differing purposes. I wonder if Luke's audience would be confused by the communal Passover meal and decided to simplify the story.
See his interaction with the Samaritan woman two chapters earlier.
Techincally, the second time Jesus tells Peter to "tend" his sheep, but the practical meaning seems to be the same.