Because Roy is dark-skinned enough that it's difficult to parse him as anything but? (Not impossible, but I feel like a lot of, say, Indian people who become internationally recognizable tend to be paler, because of the interplay between culture, socioeconomic stuff, and geography.) And aren't there bits of book commentary that specify Roy as black?
And I think lumping all the non-white ethnicities as interchangeable could lead to some... uncomfortable implications. And more to the point, implications I imagine Rich himself would find very awkward at best.
By way of analogy, Ricardo Montalban, a Latino man, playing Kahn Noonien Singh, an Indian man, is just... weird. (Yes, product of its time and all, but still weird.)