Without subscribing to anything, my answer would be that Luke never heard of Herod massacring all the children of a certain age (because that didn’t happen, whereas there were censuses, even though not the kind that require huge sections of the populace to take long pointless journeys), and figured it was too well known that Jesus’ family was originally from Nazareth, not Bethlehem.  Also, how can a star point to a manger?  And Egypt is a really long walk, and why would they come back after all those years, if they were doing okay there?  Herod was dead, but his son Antipas (the one who had John the Baptist’s head on a literal platter) was still around, and wouldn’t he know about this prophecy too?   

It’s a lot of plotholes.  

John very likely had access to earlier gospels, and he went in an entirely different direction.  The gospel authors all felt at liberty to diverge from what had been written before them.  They are not historians.  They are writing religious narratives, drawn from real events, but feeling quite free to heavily embellish them, and to add other material to them, because their audience isn’t asking for documentary realism.  (If we’re being real, nobody was insisting on that in the First Century, and it’s not so popular today.)

Please note, early Christians were no more or less stupid than people today.  They saw the contradictions.  And yet these four very different accounts of Jesus’ life (that all agree on certain points) ended up becoming canonical.  Other books didn’t.  Some were lost because they weren’t copied often enough.  

Luke agrees with Matthew (and Mark) a lot.  We tend to focus more on the disparities.  People didn’t read a new religious text and see it as Infallible Holy Writ–that happened much later.  There were discussions, debates, leading to compromises, a tacit agreement to overlook certain differences, focus on commonalities.  The point was that all four books did a good job promoting belief in Jesus, differently as their four authors believed in him.  They were all powerful stories, and stories are what keep a religion alive, make it grow.  

Matthew was undoubtedly correcting things he didn’t like about Mark.  Mark may, for all we know, have been correcting things he saw wrong with earlier accounts we can’t read now.  Somewhere behind all of these stories is the real story.  

Referring to the history of human evolution, Loren Eiseley once write, “It is as though we stood at the heart of a maze and no longer remembered how we had come there.”

All history is like that.