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?
Marketers are people, with biases, likes and dislikes. Their agendas always come through.
Nah.
A door wreath won't match my mezuzah :)