“Why do you think there aren’t a lot of black VAs?”
Now this depends where you’re looking. I only have experience on GWA/NSFW so I can only speak on that but here’s my two cents.

I am fortunate enough to have a neutral voice, I do not sound like I’m from a particular area.

American white neutral is unfortunately the standard for attractive voices. While other accented voice can be seen as exotic. They are more niched in this world.
There’s also the fact that blackness is unfortunately not profitable. Despite there being a huge bipoc listener base.

You all see a lot of what I look like on this bird app I keep my Reddit devoid of any colours photos for that reason.
It’s better to remain as a blank face for someone to imagine whatever they find most attractive. And for a lot of people that is not the image of bipoc creator.

Which leads me to the last part of this far too long tweet. Perhaps there are more black VAs than you think and they keep their identities hidden for popularity and to avoid harassment.