In the 1920's, jazz was still a new art form. Through the purchase of gramophone records, the general public were well aware of the names of the popular bands. But discographies were a thing of the future. Soloists or singers were not normally named on the label. After all, with the majority of buyers, it was the songs that were of more interest to them rather than the band. The names of the singer or the soloists didn't usually appear on the label at all. Some listeners may have wondered who the pleasant sounding vocalist was, or the musician responsible for an interesting cornet solo. I would say this is why Bix and other musicians weren't generally known to the public in the 20's. Even Crosby appeared on the label as "With vocal refrain". He gained his popularity in the early 30's after he branched out after leaving Whiteman. By then the swing era had taken off and the personnels of bands were of much more interest to the public.