How come Mary Magdalene's book was left out of the Bible?

The Gnostic Gospels - the books of mary magdalene, thomas the twin, philip, judas etc. etc. were rediscovered in 1945. Many of these were left out and not ped around because they were in conflict with what had already been established. For modern scholars they are an interesting look back at the from omissions and other possibilities for the narrative of Jesus' life.