They aren't actually excluded since the people of Far Harad existed. The issue is more that their story isn't very interesting in the Second Age and in the Third Age they are enemies. But I think the story of the Harradrim's fall due to their subjugation by the Numenorians during the Second Age is a story they could have told that fits their checkboxes without attempting to change Tolkien's existing culture groups as he depicted them. But I get why they aren't doing that, they are trying very hard to get people with nostalgia, rather than just trying to do something unique within the existing universe which would be creatively more interesting.