They are doing it to disambiguate between the color black, and identity Black. [0]

If your question is "why not african-american"? It's likely because not every black person is from Africa, or is American. Personally I think African-American is a poor term and Black American is better (if they're American, obviously).

[0]: https://apnews.com/article/71386b46dbff8190e71493a763e8f45a