Chinese restaurants are often owned by first or second generation Chinese immigrants who (1) aren't Christian and (2) haven't been in the US long enough to adopt local customs. Since restaurants often are marginally profitable, they tend to stay open on Christmas because there is profit to be made, especially since most of the competition is closed that day.
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.
