Excellent question. Even though Hughes chose to set his poem twenty years earlier, during the Great Depression, it is clear that the situation faced by African-Americans in the early 1950s was exactly the same, in terms of their lack of rights and the oppression and discrimination that they faced. Let us remember that laws protecting the rights of African-Americans only became enshrined a few years later, and so Hughes was writing at a time when racism existed and there was no support or protection provided by law in any way for African-Americans.
What Hughes seems to be saying is that, even though history has moved on, the situation of African-Americans has not changed at all, even though twenty years have passed. They still are without rights, suffer tremendous poverty and are given false promises. This makes the final stanza of this poem particularly relevant:
So we stand here
On the edge of hell
And look out on the world
What we're gonna do
In the face of what we remember.
Action is based on history and a remembered past. That the situation of African-Americans has not changed gives this final stanza a more focused mood of oppression and possible action in the face of this suffering.