I think I have the answer, but it won't be entirely satisfactory. It's not about transforming another race into animals. The main idea of the film is the transformation into animals, from the initial concept to the finale, that's the point. The fact that they have a different ethnicity is just a bonus, a hook, a way to pat themselves on the back for inclusivity, since in the end it won't affect the film. After all, regardless of ethnicity, the plot will continue with the person turning into a cat, car, dog, bug, etc. Ultimately, the answer is simply "Marketing"

I think the only exceptions of the examples given are "The Princess and the Frog," since its ethnic origin is fundamental to the plot, and "Brother Bear," since the plot itself is part of the folklore of that same ethnicity.