"In an excuse that carries a whiff of that old “the SNES doesn’t have the technology to render blood” chestnut, Sega has explained that the WiiWare platform lacks the chops necessary to replicate the CD-quality sound of the technologically-mindblowing-circa-1993 original – so, no Sonic CD for Wii owners." - GamesRadar
When I heard that Sonic CD was getting re-released for download platforms, I knew the Wii wasn't going to be included. I figured the file size would just be too big, and that indeed is the case. That's a shame, because I consider Sonic CD to be one of the best, if not the best Sonic game! Thanks to TimmiT for the heads up!