Fowler Explains why the Film didn't Feature Sonic's Friends
For this first film, we really were just looking at the 1991 game and just see where it all started and just keep it simple. Just try to nail Sonic and Robotnik and just set up their rivalry. There's a lot of great characters in the Sonic universe, but it's the most important thing is just to get Sonic set up and just tell a little bit of an origin story with him, and just do it in a way that really makes everyone fall in love with him as a character and just be rooting for more.
A sequel would feature more Sonic characters like Knuckles.