Unconscious bias basically.
I suspect many people originally thought that it was a black community crime, or a gathering that got out of hand.
But that's just nonsense - they were two sisters having a birthday drink together in the park and were utterly blameless.
So yeah I think the lack of coverage is because the victims were not white. No-one would immediately think "gangs" if two white middle aged sisters were killed on Hampstead Heath.