Like all of my theories, this is coming from a Nordic perspective and will focus on Europeans and Africans, however the ideas here will certainly be applicable to other races.
Black male, white female (BMWF) pairs are almost always seen as more appealing than white male, black female (WMBF) couples. Why is this?
Men and women are the result of sexual dimorphism, that is the biological differences between the two sexes. For example, men have more muscle mass and are taller because they are the sex designed to hunt, fight and protect their tribe. Dimorphism is needed to ensure a tribe has individuals that excel in their role.
Sexually dimorphic traits in men include being tall and muscular, but also include features like darker and tougher skin, darker hair, broader and flatter noses, and a large penis. These are our masculine traits. Sexually dimorphic traits in women are the opposite of these: being curvy but slim, smoother and lighter skin, fair hair, smaller noses, wide hips, breasts and a vagina. These are our feminine traits.
All races have these differences, but collectively men and women of one race will have less dimorphism than a group of multiple races.
European women are very feminine, with their blonde hair and fair skin. On the other hand, African men are very masculine, with a low body fat percentage, dark, tough skin and large penises. They both represent a high sexual dimorphism amongst humans. Paired together in a sexual scenario and we have a very attractive and ideal combination. A BMWF pair is the most masculine man mating with the most feminine woman.
European men are darker than their women, but are nowhere near as dark as Africans. Vice versa for African women. The result of a WMBF pairing is an inverse of the ideal BMWF pair, where the man is almost as phenotypically feminine as the woman. This is very obvious in the modern world, where many European males now lack the masculine dimorphism, being skinny, short and lacking a large penis.
Media knows this, and so they opt for BMWF couples to attract more people to their products. This further reinforces the association of darkness and power with men and fairness and elegance with women. So much so that upon seeing anything other than BMWF, viewers are disinterested.
This is not a trend, but an exploitation of our biology. We are only seeing the beginning of a permanent genetic and cultural shift.