It isn't, but only if you look at the structure of the movie. Structurally it's a heist movie set around the Christmas holiday. There are few of the typical themes you see in Christmas movie - kindness, generosity, faith, family. You could say the reconciliation of John and Holly could be an example of family, but it's secondary to the heist plot line. Also, it was released as a summer blockbuster and it wasn't marketed as a Christmas movie. It wasn't really widely considered a Christmas movie until social media grabbed onto the Christmas backdrop and related gags and TBS (or whoever) started showing it every year.
ON THE OTHER HAND, I watch it every year at Christmas. It's probably my favorite movie and I'm glad it's become an annual tradition for future generations. Christmas movie? Sure. Action Thriller? Yup. One of the best movies on the 20th century? HELL YEAH.