…Because Dark Sonic, despite only appearing in for like one second, is still one of the most know part of the show, to the point where people have repeatedly asked if SEGA has any ideas of reintroducing him, not just because people think he's cool, but because people think there's alot of untapped potential within the idea of Dark Sonic. It would also be another way to make the character more diverse.
Maybe instead of Sonic going Dark only with 2 or 3 emeralds, he turns into this sort of pseudo-Dark Sonic as seen here, like in my suggestion:
But when he gets all the emeralds, he can go true Dark Sonic and Super.