Here's why:
* Classic Sonic's gameplay is different: Classic Sonic in Sonic Generations uses the gameplay mechanics from the original Sonic games. He has the Spin Dash and can jump, but he doesn't have the Homing Attack.
* Modern Sonic's exclusive move: The Homing Attack is a signature move of Modern Sonic. It allows him to lock onto enemies and dash towards them, while Classic Sonic doesn't have this ability.
However, you can still enjoy Classic Sonic's unique gameplay and classic levels in Sonic Generations. He's still a powerful character with the Spin Dash and other abilities that make him fun to play.