I’m not going to claim to know all that much about the history of radio genre classification and segregation, but I do remember when I was a kid (white and parents mostly played “black” music) there were a lot of hit songs and artists on “black radio” (and you could say the same for MTV/videos) that did not “crossover” to the pop charts for whatever reason, and it seemed race or at least image of the performer had something to do with it. I think THAT reason may have less to with the way things are separated now, but there are certainly categorizations that make little sense. If anything now, with pop supposedly more “rhythmic” oriented, I actually see a lot of white POP (and dance music) artists played only on alternative and rock stations. It still has a lot to do with image. A lot of it is time/era difference. I find it hard to compare the categorization of N Sync and Motown (at least if it wasn’t the Motown label of the 1990s/2000s, which I’m assuming was not meant), just 2 completely different eras. And FWIW, much of Motown was considered pop and N Sync had R&B play. A better question might be why N Sync was considered pop and on MTV, while black boy bands of the era with similar styles were limited to BET/R&B. Or, why do rock stations play Beastie Boys and Eminem but no other similar music?