Cultural preference. In some countries, particularly those that are traditionally Catholic, gift-giving is ordinarily on January 6 precisely because it is the feast of the Epiphany, with which gift-giving is more strongly associated because of the Magi’s gifts to the baby Jesus. In other countries, such as the United States, where Christmas is more removed from its Catholic roots, December 25 is the day of gift-giving because of the legend of Santa Claus who is said to distribute gifts on December 25.
This doesn’t mean that Catholics in the United States can’t give gifts on Epiphany if they prefer. Gift-giving is a family custom and not something that is regulated by the Church.