I think part of it is that the market for Black lawyers in most cities is saturated. Black men having a strong entrepreneurial spirit realize that opening up their own firms is likely to be incredibly difficult. Additionally, the prospects of making bank if you have to work for someone else are better, on average, for a Black man going to business school rather than law school. Finally, I think there's been a stronger push for the Black men with social justice in their hearts to go to med school rather than law school. This reflects the strong emphasis put on math and science on African Americans these days.
I also believe that there is a tremendous amount of indirect and subtle racism polluting the profession.