This would probably be a good place to start: http://tolkiengateway.net/wiki/Racism_in_Tolkien's_Works But essentially it's because of institutionalized racism and the view of white people as the default race. I'm sure the casting directors rationalize it by claiming that it's "historical accuracy" and therefore they can get away with claiming that there's only one skin color (you'll notice that sometimes when people try to draw parallels between Middle Earth and the real world, some people will say "THAT DOESN'T MAKE SENSE IT'S AN ALTERNATE WORLD NOT BASED ON EARTH AT ALL" but then as soon as someone says the series should have more diversity in it they cry historical accuracy).
With the exception, of course, of the villainous lackeys or "bad" races. In the books, the orks are described as being dark-skinned and there's also an entire race of people (mentioned in passing I believe) described as dark-skinned, slanty-eyed, cruel, and barbarous. But of course we can't have a single nonwhite person among ranks of the Good races of humans, hobbits, dwarves, and elves.
Tolkien might have been "a product of his time" but that doesn't make him any less racist. Racism is not comparative.