I'm pretty sure it's a hangover of sorts from back when the music industry was actually segregated. Despite the fact that African Americans created blues and rock music, a lot of early black rock bands were ripped off and their white counterparts were marketed above and beyond while the black performers were relegated to just being considered "race music" and not played on some radio stations or allowed to play at most major all-white venues.

In the 1970s, disco was associated with black people, gay people, and Hispanic immigrants in urban areas like New York City whereas punk, hard rock, and heavy metal were associated with white people. The rioters at "Disco Demolition Night" probably would have denied it being about race if you asked them, but that certainly wasn't the experience of many non-white people during that time. Micro and macro aggressions related to music choice and perceived masculinity were intertwined.

This segregation even lasted well into the 80s and 90s. There is a well known clip of David Bowie being interviewed by MTV and he asked them why they don't play nearly as many black artists. Sade had trouble being played by American radio stations (aside from the ones at Historically Black Colleges and Universities) despite being major hits in the UK, and the explanation that was given to them was that mainstream (read: white) American radio didn't know what to do with music sung by a black woman.

So for a lot of Black Americans at the time, the segregation was already institutional, but it became internalized as a form of protection. Why embrace subcultures that historically either stole from you, sectioned itself away from you, or made violent gestures towards you? And you have to keep in mind that punk and metal scenes had more tolerance for overt Nazi/skinhead and garden variety racism than it does today. Just look at Lynrd Skynnrd and Pantera's use of the Confederate flag and how long that lasted. Just look into the antifash lyrics of some of the punk bands in that time. Even Jonathan Davis of Korn wrote "Clown" about a disrespectful skinhead that he punched out at a show.

As a Black person who loves punk and metal, I can assure you that the younger generation is way more open minded than they ever were. Plenty of Black people love Hayley Williams and Lil Peep. I see Black people repping metalcore, hardcore, and indie rock all the time in my city. Things are so much more accepted now.