Well, that makes sense, since before B&W the only differences in game was the legendary & evil team in Ruby & Sapphire, whereas Black & White has Opelucid City and several other locations which were completely different between games, tying into the Yin-Yang theme, which wouldn't have worked for Grey.

Also, as a handy note, there's no such thing as "Ying-Yang." It just has the one G.