Because in America, strong-minded black women are SCARY.
Society does a good job of being two sided, especially when it comes to race. When a white woman is called strong, usually there are positives connotations associated with it. Words such as "independent," "powerful," or "influential" are often heard in the same context. However, when it comes to black women, strong means scary. Society is intimidated by the sight of strong minded black women. But why? Is it because of one experience that caused this reaction towards all black women? Is the only difference skin color?
https://medium.com/i-do-see-color/stop-calling-black-women-intimidating-3b89f8729130