You are probably American, so it might be a shocking reveal to you... the various human races don't originate from the same continents, y'know. It makes sense that in a fantasy world where races don't mix as much as they do now that we have ships and plans that can get you from one end of the world to the other in days. You cannot just hop on the back of Smaug and ask him to take you from Gondor to Umbar.Perhaps if every character in Middle Earth were black, it would strike you as odd and in need of more variety?You don't need various sub-races when you already have SPECIES diversity... in a world of orcs, elves, dwarves, humans, dark elves, halflings... people can hardly complain about it being "'whites only". That's like saying "Romulans are white only" - no they aren't, they are Romulans.
PLus, we do have variety - we have humans, hobbits, dwarves, and many races of elves (of course it might be argued they are more like separate elf families, considering their longevity) and various races and sub-races of orcs from small goblins to huge uruk-hai.
In LOTR, we do have black people - they live in the south, in Harad. Not many of them live in the places where the story took place, though.