I don't understand how, as one of the most diverse countries in the world, we can still suffer from such horrific, institutionalized discrimination and racial issues. Perhaps that is just part of the reason. It seems to me that while other countries have similar problems, our society in particular has a very violent streak to its systemic racial problems. I can only hope that, one day, we can overwhelm them, but I am not as optimistic as I used to be...
The US is only really diverse if you reduce everyone into a paste and spread that paste evenly across the entire country. You look down at the level of counties / cities / neighborhoods and the US really isn't that diverse. Which on the one hand is discouraging, but I think you could also argue the US has never really tried actually having diverse communities, and that that therefore could still work.
I'd say in comparison to a lot of other Western cultures we are pretty diverse - having been around in Europe, while racism is perhaps much more subtle and not nearly as violent here, the population definitely tends to be much more homogenous. I think it probably comes down to the fact that there is no such thing as ethnically American unless you're Native American, whereas other countries have a strong sense of ethnic nationality.