Jesus said to him, “Have I been with you so long, and yet you have not known Me, Philip? He who has seen Me has seen the Father; so how can you say, ‘Show us the Father’?"- John 14:9
In the later half of the first century AD Christianity really began to enter into the mainstream, the Great Temple in Jerusalem had been destroyed in 70 AD which Jesus had prophesied happening nearly 40 years earlier. The correct prophecy of Jesus drew in a lot of new interest into Christianity and with it, a lot of new viewpoints as people began interpreting the text for the first time. Among them were the Gnostics, they were a group that first gained steam in the early to mid 2nd century (100s AD) who claimed, among other things, that Jesus and God were not one in the same but separate beings. The Gospel of John was likely written in response to the early stages of Gnostic thought developing. The three other canonical gospels (known as the Synoptic Gospels) tell largely the same story about Jesus three different ways, these were not the only three Gospels written in the first in second century, in fact we know of dozens more (some of which survive in fragments).
Now contrary to popular belief there was no single council or group that decided Biblical Cannon, at least not for many centuries. In a time before the printing press, when everything had to be hand written, only the works about Jesus deemed the most important or most accurate by the general audience were widely printed and distributed. That's why the Gospels of Matthew, Mark, and Luke were so widely written and publish because they told basically the same story about Jesus most 1st century AD people agreed on. The Gospel of John was a bit different, it tells a largely different story with the author, Jesus's disciple John, explaining his thesis in the beginning.
" In the beginning was the Word, and the Word was with God, and the Word was God. 2 He was with God in the beginning. 3 Through him all things were made; without him nothing was made that has been made. 4 In him was life, and that life was the light of all mankind. 5 The light shines in the darkness, and the darkness has not overcome it.
6 There was a man sent from God whose name was John. 7 He came as a witness to testify concerning that light, so that through him all might believe. 8 He himself was not the light; he came only as a witness to the light.
9 The true light that gives light to everyone was coming into the world. 10 He was in the world, and though the world was made through him, the world did not recognize him. 11 He came to that which was his own, but his own did not receive him. 12 Yet to all who did receive him, to those who believed in his name, he gave the right to become children of God— 13 children born not of natural descent, nor of human decision or a husband’s will, but born of God.
14 The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the one and only Son, who came from the Father, full of grace and truth."
Basically as you can see the entire thesis of the Gospel of John is that Jesus was God. John's whole purpose for writing is not to tell you the full life story of Jesus, but to highlight key sayings and actions of Jesus that proved he was God. Given when it was written (around 90 AD) it was likely created by John in response to the development of Gnostic thought. John and Jesus's early crew believe fully that he was God and John wanted to set the record straight before he died (he died around 100 AD, the last of Jesus's disciples to die). While the three synoptic Gospels survived because they were seen as the "most correct" story on Jesus, the Gospel of John likely survived despite deviating a bit from the main story because the author was one of Jesus's disciples, and because it's thesis was one the early Christians agreed on, that Jesus was God. As a matter of fact this was something Pliny the Younger, the Governor of what is today Turkey, noted in 111 AD when writing to Emperor Trajan regarding his persecution of Christians.
"They asserted, however, that the sum and substance of their fault or error had been that they were accustomed to meet on a fixed day before dawn and sing responsively a hymn to Christ as to a god".
