Carnegie definitely tried to keep a diverse workforce in his steel mills as part of his strategy for fighting unionization. I'd expect the main driver would be

Carnegie definitely tried to keep a diverse workforce in his steel mills as part of his strategy for fighting unionization. I'd expect the main driver would be different groups within the workforce feeling that they're different sorts of people hurting efforts to band together, for which it's mostly subjective diversity that would hurt. The sort of thing where a mix of old-stock Americans, Irish-Americans, and Italian Americans would have considered themselves racially diverse in 1920 but not in the modern day. Other potential things would be different languages, different notions of fairness, attending different social organizations, etc.