I think that the white men who do these ads might have a hard time envisioning themselves marrying a black woman. If it's a woman marrying an "other", then it's fine because they don't really identify with women necessarily, but if it's a man then they identify, and it makes them uncomfortable to imagine being with a black woman.

That's just a hot take though. Another take: you might want to compare the traditional negative stereotypes associated with black men (violent, impulsive, stupid) to the stereotypes associated to black women (motherly figures, vulgar and hypersexual). Deconstructing the first stereotypes might be easier to do for an advertiser, because they don't confront any internalized mysoginy.