If the wife of an emperor, king, or powerful merchant gave birth to a child, it would have been widely known and celebrated. Everyone would know where that child was living.
But almost no one seemed to know Jesus was born. Herod would not have known if the Magi hadn't told him, and the Magi wouldn't have known if God hadn't told them by the use of a star. The shepherds would not have known if the angels hadn't told them. Even after Herod knew the Christ had been born he still was unable to find the child Jesus.
Jesus was born to such an unknown inconsequential family that He blended into the mass of humanity wherever his family lived. With few exceptions no one seemed to ever suspect this child born to Mary and Joseph was anything but just another face to feed in the mass of Jewish peasants.
Jesus' existence was so nondescript that even today some scholars question He ever lived. Think of it: He was virtually unknown until His baptism by John, and then only personally known in His local region in a small country in the Roman Empire. Even then His period of public notoriety was short lived, about 3 years, and he was executed with criminals as a law-breaker.
His short rise to fame was off the tracks people used to become famous. He didn't go to any famous schools or seminaries. He wasn't involved in the politics of His country. He wasn't in a scribal college and He was “unpublished.” He didn't seek license to teach... he just did it.
In a world where duplication was done by hand, 3 years is barely enough time to complete the publication of just a few copies of the biography of a famous person's life... but the life of a Jewish peasant?
Sure... there were writers who wrote about His life: Matthew, Mark, Luke, and John. But if no one valued their writings then there would have been no scribal duplication of their books. But the writings of these men became the most widely translated and widely read publications in human history. People like William Tyndale have died trying to get these writings into the hands of the masses.
Jesus' fame spread in the same way it generally does today: by one person telling another person. This is one of the most miraculous realities in all of human history. The fame of this virtually unknown Jew has been kept alive because people talk about Him. Jesus was and is a nobody only to people who don't know Him. But when you come to know Him He is somebody who must be talked about. There has never been somebody like Him.
People have tried to eradicate His fame from popular culture over the centuries only to see His fame spread more widely by their efforts. They find it is more like trying to put a fire out with gasoline.
How could this happen? Why is He so famous? Because of who He is, what He said, and what He did. If you don't know much about Him I suggest you begin your reading with the Gospel of John or with His Gospel.