I don't agree that ethnicity is the main factor. I believe that the reason these girls weren't believed is because of misogyny ie seeing women and girls as less than.
In her report, Casey talks about adultification which has been a problem since time immemorial. Treating girls like they're much older than they are and not seeing them as children. She describes the victims being treated as though they consented, when children legally can't consent. One social worker described a 10 year old as a "prostitute" - that's not prostitution, that's child sexual abuse.
These girls were treated as "trash" by the people meant to be safeguarding them and many were prosecuted. This is a reflection of the misogyny deeply ingrained in society which has a rape conviction rate of 1%.