There are actually several reasons I came at the idea of The Dispossessed as being multicultural.
First, the notion was as much about the US' recent cold-shouldering of asylum-seekers as it is about systemic racism. As the event goes on, it becomes clear that the gang are essentially stateless persons trying to find a place in the world for themselves. It is that, even more so than material things, of which the members have been dispossessed.
Second, in-story, there couldn't have been enough Wakandan exiles to staff an nation-wide crime ring. Since there weren't, Killgrave has guided the exiles in using this statelessness as a common bond in organizing the gang. That's not without precedent, what we thought of as organized crime in the 1910-1940s often involved cooperating ethic groups (Italians, Jews of many national origins, even African-Americas). Further, the number of Wakandans involved needs to be relatively small, because otherwise, there's no way the expulsion could have been kept under wraps. That would mean, on at least some level, T'Challa bares complicity for the circumstances, rather than sovereign responsiblility for it. We have to be able to believe he could be kept in the dark until shortly before accepting Captain America's invitation to join the Avengers. You want Black Panther - and by extension, Wakanda - to take some moral excoriation here, but don't want to outright demonize him.
Third, issues of US systemic racism generally focus on African-Americans, and for good reason. The sources of oppression against African-Americans are unique, but systemic oppression is not unique to African-Americans. For whatever reason, Latinos have been less able to cast a light on their struggles. In-story, the gang doesn't want its driving core philosophy diluted by other motives (i.e. the view of so many striving African-Americans to view themselves as part of a common experience), so limits the number of non-immigrants it takes into the organization. That has the benefit of letting us see some of the flaws in this group; their grievances may be real, but their goals are about themselves, not any greater good. Killgrave is the focus of this mentality, but he's hardly alone in it.
Finally, and perhaps most importantly, you don't want The Dispossessed to be All-African because it strikes me as a bad idea to make it too easy for readers to characterize The Dispossessed as the IRA to BLM's Sinn Féin. There are already enough distasteful people out there characterizing people protesting for their lives and rights as criminals.
Thank you.
I would be honored.