Because diversity doesn't equal number of immigrants and/or skin colors. What's more "diverse", when your neighbor has identical culture but a different skin tone or when he doesn't even speak the same language?

In both ethnic and linguistic diversity rankings US doesn't even reach top 50: https://en.wikipedia.org/wiki/List_of_countries_ranked_by_ethnic_and_cultural_diversity_level

Go tell that to Indians who have over 120 major languages and more than 2000 ethnic groups that they're all the same because of skin color.

The "race" as US understands it such an arbitrary concept that virtually no other countries even recognize it.