Why is Howard University still predominantly black?
I just don't think HBCUs are all that unusual in terms of having a student body that's not a random demographic sample of the United States.  Brandeis has a higher than average percentage of Jewish students, Cardiff has a higher than average percentage of Welsh students, and Duke has a higher percentage of southern students.  Students pick schools in part by cultural and social match, and ethnicity plays a role there.  Furthermore, people go to schools that their family members and friends went to, and a black kid is more likely to have grown up hearing "keep up the good work and you'll get into Howard/Morehouse/Spelman" from their alumni parents and teachers while a white kid's role models are less likely to be Howard alums.  Finally, demographic shifts happen slowly: Sarah Lawrence has been coed since 1968 and it's still 3/4s female.