First, you'd have to answer what is the value to post on a Black-owned forum. Black people don't hold power in this country. Therefore, posting on Black-owned media would be like preaching to the choir. At least, in the last two decades, when Black people post on white-owned sites, they've elevated their professional status. Many Black activists, entertainers, journalists, even educators have launched their profiles through Facebook, MySpace, Twitter, and now Instagram. Does this serve Black people? Nope, but it does help some Black people to realize their individual dreams.
And speaking of individuality, just like any other ethnic group, we all have varying views of the world. Some of us are liberal, some of us are religious-fundamentalists, some have patriarchal views, others have matrilineal perspectives, etc. So a Black forum doesn't serve Black people either. At least not all. Humans tend to flock to communities that connect them mentally.
For example, This community serves us through literature. We visit here because of the books written about Black people and books written by Black people.
Also, when we did congregate on Black-owned media, especially book shops or through book publishing -we were targeted by the FBI. So maybe black people have been programmed into believing it is better to infiltrate white-owned media and hide in plain sight, so to speak. This way, if we are targeted, it is in full view of the world.