Christmas carols and Mary

Can anyone explain why so few Christmas carols mention Mary? I looked at a list of carols and...

these mention Mary:

1. Angels we have heard on high
2. God rest ye merry, gentlemen
3. Once in royal David's city
4. Mary's Boy Child

and these do not mention Mary:

1. Away in a manger
2. Did you hear what I hear?
3. Go tell it on the mountain
4. Hark the herald angels sing
5. It came upon a midnight clear
6. Joy to the world
7. O come all ye faithful
8. O holy night
9. Silent night
10. The first Noel
11. We three kings
12. While shepherds watched their flocks

I find it amazing that so many Christmas carols do not mention the Mother of Christ.