So, then, if Christmas is a subgroup of the "generic" holiday, why is Kwanzaa and Hanukkah not also part of the generic "holiday"?
Probably because there is limited specific merchandise geared to those relatively small groups. A door wreath, for instance, could be used by a Jewish or black family. A Menorah is likely to only be used by a Jewish family.
Retail search engines only have one purpose: to move merchandise. That's it. No political agenda.
What's more, it would be foolish to promote a political agenda via a search on a retail website. What? Someone is suddenly going to be steered to a Kwanzaa item and become black?