I believe Cyrillic а and Latin а are different because there already existed legacy encodings where а and а were considered different characters, so Unicode kep

I believe Cyrillic а and Latin а are different because there already existed legacy encodings where а and а were considered different characters, so Unicode kept the distinction for backward-compatibility. While there were no existing legacy encodings allowing to write Chinese and Japanese at the same time, so there was nothing to keep compatibility with.