Why is Luke's gospel called the gospel of infancy?

Not whole his gospel is called that name. The first chapters held this name because as the only one of four gospels that are in Bible tells anything about annuciation (of both St John the Bapist and Jesus), birth, epiphany, escape to Egypt and presentation in the Temple. It also mentions about Jesus aged 12 discussed with Jewish wisemen in the Temple. The next chapters tells same story as 3 other gospels, but not exactly the same - in other Words, with another details and so on, but meaning is the same.