There are at least a couple of differnt levels here.
Why are the Gospels different?
The Gospels, Matthew, Mark Luke and John are core to the Christian faith.
Without the life and person of Jesus it is pretty certain that there would be no such thing as Christianity, either in name or in character. For those of us living after the Biblical age the four gospels are the only record we have of that, and it makes them collectively exceptionally important - foundational - to Christianity. If we had never had certain other books of the Bible , Christianity would probably be very different. If we had never had the Gospels (or their equivalent) it would never have happened.
Why is John different?
Three of the Gospels (Matthew, Mark Luke) are called 'Synoptic'. They present a similar view of Jesus, and most of the stories in them are found in at least two of those Gospels. It's reasonable to argue that the picture we have of Jesus would be only somewhat changed, if we lacked any one of them. John, on the other hand, presents a very different (not contradictory) picture. If it was missing we would have a significantly different view of Jesus. It also presents an unashamedly 'evangelistic' view of Jesus, presenting not just the story but making explicit statements about his significance. For example it contains the famous "John 3:16" verse.
For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.
In my experience Christians often give out Gospels because they contain the core of the Christian message. They are also very readable and relatable. You can read one of the gospels in around an hour, and at the end of it you will have a pretty clear picture of the main points of Christianity. John probably gets used more often than others for the reasons above, but not exclusively.
At the time of the Vikings, any written copy of a Gospel was rare, and so considered precious by Christians (for its content, not its monetary value, though that was substantial) and it is that that makes Athelstan value it. It is entirely possible that John was the only Gospel Athelstan owned, and that he would have been equally attached to any of the other Gospels. When Athelstan says "Because without it there is only darkness" he is not referring to the physical book, but to the message written in it.