IGN. Ono apparently stated that incomplete 3D models were created of the characters, and the staff even planned out T. Hawk's combat, but no explanation was given for why the pair didn't make the four-quel. The characters are the only two fighters from Super Street Fighter II Turbo missing in SF IV.
Ono expressed that if there's enough support from the fans, the characters will be added at a later date. It's not clear if that would be DLC for the current game, or if they'd be added in a later iteration, like, Super Alpha Street Fighter IV Turbo Remix Deluxe.