Well, "Christmas Vacation" is rated PG-13 because it contains some mature themes and language that might not be suitable for younger viewers. The movie features a lot of adult humor and comedic violence that could be considered inappropriate for children. Additionally, there are some scenes that depict drug use, which is also a reason for the PG-13 rating. However, despite these factors, the movie is still a classic holiday film that can be enjoyed by the whole family if watched with caution and discretion.