Sonic CD was recently announced for digital rerelease on just about every possible gaming platform, with the exceptions being the Wii and the 3DS. No word yet on why the game is skipping a 3DS rerelease, though it's a safe bet that Sega is trying to avoiding competing with themselves in the 2D Sonic market with the upcoming Sonic Generations on the 3DS. As for the Wii, word has it that Sonic CD is skipping WiiWare because of the service's 40MB file size restrictions.
Sonic CD is over 80MB in size, so Sega would have to do some serious compression to get the game to fit on WiiWare. I'm sure they could squeeze the game down if they thought it was worth it, but chances are that no one would choose the compressed, potentially compromised WiiWare version of the game when it's hitting so many other platforms without the need of compression or compromises.
If you really want to play the game on your Wii, just pick up Sonic Gems Collection for the Gamecube. It's worth it.