We are different in economic and social class, religion, politics, urban/rural, interest in sports, interest in music, gender, sexual orientation, ethnicity and race.
And yet, when we talk about diversity race trumps all other forms of diversity. It is the most visible (other than gender) form of diversity. All other forms of diversity requires getting to know the person.
When it comes to discrimination, again, the most visible form of difference is the most common basis for discrimination.