Why does only the Gospel of John speak clearly of Jesus' divinity and not Matthew, Mark, or Luke? Why is it only the latest of the Gospels to be written that reveal the divinity of Jesus?
Jesus' Divinity in the Gospels
Well, actually, the other Gospels likewise present Jesus as bearing a transcendent status and significance. To stay with Mark, there are the various demonic recognition scenes, and Jesus' actions that allude to divinity such as walking on the lake, et alia.
The difference isn't what is affirmed, primarily, but HOW it is affirmed/expressed in John. John is much more explicit and foregrounds Jesus' divine status more.
The Synoptic Gospels were focused more on presenting Jesus in the genre-character of a great teacher, with more "biographical" character. John openly relates Jesus' ministry through the lens of "post-Easter" insights believed to have come via the revelatory activity of the Spirit of God (as related in John 14--16).