My guess is that if he was asked why he hadn't written more characters who are black, he would probably say because he would be afraid that his writing would seem false. Burt Shevelove told a story that when Smiles of Summer Night was being considered for source material, Sondheim went to him and said that he wasn't sure he understood enough about turn-of-the-century Swedish people to write it.
Sondheim said that the reason why he stopped working on Muscle was because it required a contemporary sound that he couldn't provide. If he had tried, it would have sounded false.
Obviously, he has not limited himself to writing only about Americans. Perhaps Night Music did give him more confidence to write about other cultures and countries.
Yes, there was Forum before that, but that's written in a basically American style. They tried to avoid anachronisms but they weren't trying to write something that would reflect real life in Ancient Rome. And while there are some Italian characters in Do I Hear a Waltz?, it's about Americans in Venice. And he wrote it reluctantly.