Are you under the impression that race and nationality are equivalent? If you’re asking whether the term is considered dehumanizing, that’s been answered for you, and if you’re asking why, that’s been answered as well. In English, racial and ethnic terms are generally used as adjectives, and we don’t use adjectives as nouns when referring to groups of people.
Brazilian isn’t a race, and Black isn’t a nationality. Incredibly, it’s even possible to be both. I hope your confusion has been cleared up.