Anybody able to explain the curious observation that the black characters didn't get last names?

I know there are several minor characters who did not get last names, but more main characters like Cinna also didn't, while most other main characters did.

Also, it seems clear that Thresh and Rue came from a District that was primarily black, while Katniss and Peeta's district was primarily white. So, it does appear that, in this dystopic world, there is division across the districts. Having said that, I know that there will be a rebellion which suggests some kind of unifying of the Districts against the Capitol, but I've only seen the first movie thus far.

Maybe there's nothing to it but, if there is something to this, I'm curious what it is.