Others have mentioned that it's because Chinese places are open, but another reason is that Chinese food is (or was) not obviously treif, meaning that there is (or was) a degree of plausible deniability around eating it.
Source: family apocrypha.
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.