Because SRB2 isn't designed as a cooperative game. It's designed to emulate the classic Sonic games in 3D, which are mostly single player affairs. Sure, there was essentially a cooperative mode in the game by having the second player control Tails, but that really WAS cooperative because player 1 moved the camera and it was therefore not possible for player 2 to go off and explore on his own.
There are games that ARE designed around multiple players supporting each other to defeat challenges, but classic Sonic isn't one of them. If you'd like to make your own Coop-required level pack, you're free to do so and the support is in the game, but it's not something that we're interested in doing ourselves.