All of the evangelists had their own principles for the selection of incidents in our Lord’s life about which they wrote. John explains his here:
“Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in his name” (John 20:30-31).
When John wrote his Gospel, the synoptic Gospels had been in circulation for quite some time. So John felt free to expound on the theological meanings of many of the incidents in our Lord’s life and also felt free to leave out incidents covered in the synoptics. Thus, Simon of Cyrene was left out of John’s account and John concentrated on Jesus’ discourses at the Last Supper rather than the institution of the Eucharist.
Particularly in the case of the Eucharist John probably felt free to concentrate his energies elsewhere because all of the synoptic evangelists and the apostle Paul had recorded the Eucharist’s institution before John wrote the fourth Gospel.
As for John’s notation that Jesus carried the cross by himself, he did carry it himself before Simon was conscripted to help him. John simply didn’t choose to cover Simon’s part in the story.
Consider this analogy: In baseball, the win is credited to the pitcher who opens the game, assuming he lasts the required number of innings, even if the last innings are handled by someone else. So it can be said that Jesus carried the cross by himself even though Simon helped him in the latter portion of the trip to Calvary.
