Sewer Shark wasn't originally made for the Sega CD in the first place. It was originally made for a system that used VHS tapes instead of cartridges or CDs. Hasbro, who developed the system, decided to drop the idea, may be for money reasons, or something else. So when the time came to release the Sega CD in the U.S, Sega probably decided to release this game as a launch game because it featured FMV and Sega wanted to show how powerful the Sega CD was through FMV. Thus, Sega bought the rights for it and published it under Sony.