Greetings!
I thought this is an interesting topic.
You can accept the traditional view that the Apostle John wrote the Gospel of John and so go into all sorts of speculation as to why it differs from the Synoptic Gospels (Mark, Matthew, Luke)
OR
You can recognize that the Gospels (and really the Bible entirely) was not composed and compiled in the homely fashion that the Society/Organization and even other Christian churches/relgions portray, namely that a sole (or in some case a few) penmen were sitting peacefully at their desks labouring under Divine Inspiration.
The real story of the Bible is that it has ALWAYS been a product of politics and dispute, of factions consisting of winners and losers, of rival priestly houses and sects and ultimately of competing "truths." Mostly, we know only the "winners" the proto-orthodox view that came to be mainstream Christian belief and essentially Christianity as we know it. There is in fact, only one certainty when it comes to the Gospels (and the Bible Canon) no one, except possibly their own authors, considered them to be the "last word" on Jesus' teachings and deeds. (The disputes about what writings should be considered sacred, authoritative and eventually canonical continued for centuries and in some respects are still not setlled.)
None of the original copies of any of the books of the New Testament (Christian Greek scriptures) exists (not too mention the original writings themselves of course.) The earliest complete text copies date only the the fourth century C.E. We have fragmentary copies of some New Testament writings dating to the 3rd Century and earlier. The earliest surviving copy of any New Testament book is a credit card sized fragment from the Gospel of John that dates to 125 CE (+or- 25 years). Additionally, by 150 C.E. the Gospel of John begins to be cited by other writers and is generally known.
Most scholars feel that John was written about 100 C.E. as a reaction to the various "heresies" that were wrestling for dominance at that time among Christians. Obviously this date makes the traditional assignment of authorship to the Apostle John problematic but not outside the realm of possibility. It seems evident that the celebration of the Last Supper or Eucharist was not uncommon by that time and was probably known to John.
Interestingly, the Didache, or "The Teaching of the Twelve Apostles to the Nations" composed about 100 CE describes the celebration of the Eucharist and gives the liturgy in reverse order of what is commonly used today, namely it describes the prayers over first the cup of wine and then of the bread. Therefore, we can infer that at least among some Christian sects/congregations/groups celebrating the Eucharist/Last Supper was common and part of their faith.
As I stated most scholars believe that the Gospel of John was not written by the Apostle but only assigned authorship to him for the sake of gaining support for its beliefs and particular version of Christianity. In reading the Gospel it is self-evident that unlike the Synoptic Gospels (acknowledged to have been composed signifcantly earlier), the Gospel of John concentrates on Jesus' work in Jerusalem. Jesus' discourses tend to be long and argumentative as woudl suit a writer in disputing priests, theologians and "heretics" of a contrary view. Also, Jesus appears far less like a parochial Jewish prophet and far more like the universal "Son of God."
Most likely, John was composed by a secretary of the Apostle or close associate. Perhaps, he did have much of the information relayed to him by the Apostle but it also appears that the author was reacting to the political situation within the Church. The fact that the writer many times stresses the favoured position of the source of the information by referring to John, or the Beloved Disciple, etc. suggests an attempt to assert the authority to speak with particular weight about Jesus, essentially, saying that these are the words of not just any disciple but the one that Jesus especailly loved.
Thus to finally answer the question you raise in your post, it appears that in specifying the liturgy to be followed in the Eucharist or celebration of the Memorial/Last Supper, the author of John was attempting to clarify and to set out a standard procedure in the face of what may have been for other Christian sects a celebration that took place in various forms. WE have already seen how the Didache set forth a different order than is found in John, so likely other Christian groups in Alexandria, Elephantine and others such areas likely had their own customs.
-Eduardo
Aside to JCanon:
Tradition assigns authorship of the fourth gospel, the three epistles of John and Revelation to the same person, namely John the apostle, that is, John son of Zebedee. (In the Catholic bible Revelation is accordingly entitled "The Apocalypse of St. John the Apostle.") The language of Revelation, while Greek, is filled with Semitic word order and idioms, and is rich in Old Testament allusion which supports the idea of a Palestinian Jew, John the Apostle, who thought in Hebrew or Aramaic and whose Greek was learned later in life.
However, the author of Revelation does not positively state that he is the Apostle John or even the "Beloved Disciple" and there is an enormous difference in style, vocabulary, and thought between the fourth gospel and Revelation. Thus the KJVersion calls Revelation "The Revelation of St. John the Divine" while the Revised Standard Version is even more cautious and calls it "The Revelation to John" while the NWT says similarly "A Revelation to John." MY own opinion is that Revelation was probably written by the Apostle John but that the Gospel of John being written a few years after his death was only ascribed to the apostle for the sake of bolstering the "proto-orthodox" version of Christianity, although it may have been written by a personal secretary or close associate of the Apostle.
