If you are Jewish, Christmas obviously isn't nearly as important to your (often nominally) Christian neighbors and there isn't much to do on Christmas day. If you decide you don't want to cook, Chinese restaurants are way over-represented in the choice of open restaurants. Once you do that for a couple years it becomes its own tradition.
I grew up in a Catholic household but my family, after years of making Turkey and spending hours cooking, for the past few years has switched to Chinese food on Christmas. I don't miss the turkey and gravy.
I came into this thread thinking that I don't really have any great Christmas memories. My family was poor growing up, my parents shouldn't have been together for as long as they tried to make it work, and my father had a lot of issues with drugs and alcohol. But now that I think about it, these Christmas Eve memories with my wife are my favorite.
Source: family apocrypha.
This is all apocrypha, so take it with a grain of salt. But my understanding is that this would be a case where marit ayin would not be a significant concern, since the Chinese dishes in question were not visually identifiable as e.g. pork.
Or another framing: if you were a semi-secular family (like mine) that tried to keep a semi-kosher home, it would be easier to eat a dish that contained finely minced pork or shellfish or similar. American Chinese food fits those parameters while also being available on Christmas, etc.