Why do they call jazz music race music?

"Race music" (race records) is a term applied to gramophone records from the 1920's and 1930's that featured music in a variety of African-American genres such as jazz, boogie-woogie, blues, jump blues, and rhythm and blues.

From 1949 to 1958, the term "race records" denoted the category of music now termed "rhythm and blues".