No idea really... but there has been waaaay too many sonic games on the Wii really. Plus Sonic Colours is overrated

this is a very very good question!!

I don't know :P haha I just think they've released too many sonic games. With Colours that's 4 main series sonics (!!), not to mention 3 Mario and Sonic games and more sonic spinoffs... 

I think they wanted to make a Wii exclusive and do seperate development for the HD consoles. Weirdly though the 3DS got generations so I find no reason why they didn't invest in the Wii

The HD version of Sonic Colors would not run properly on Wii, there is way too much crap going on. They already said they focused on the HDs because they wanted to really turn Sonic up to 11. Good choice IMHO, the game is actually way better than Colors and this is coming from a guy who loved Colors and just bought the DS version. However I dont know how the 3DS got a version and I dont know why they dont port that one to Wii, sure it wont be the same game as the HDs, but its something. They probably will port it too WiiU to take advantage of the early adopters.
A better question is why is there on SonicCD not being released on WiiWare?? That makes no sense. Maybe Sega & Ninty are not that tight anymore.

Probably because this is Sonic's Aniversary game and they wanted it to look as best as it could. Anyways not too long ago there were some rumours about a new Sonic game on Wii U in development, maybe we'll see it at E3?

Boo!  If anything Sonic Colors is underrated.

It's probably the best 3D Sonic game ever made...

It's because Sega thinks that Sonic is what the hardcore gamers want to play so he has to be on the hardcore consoles.

Which really is nothing much to me! Put aside the Sonic cycle and let's focus on the game without comparing it to its predecessors. If we compare it to other platformer games (the marios, DKCR, Kirby etc..), then I really think it lacks a lot.

It's a decent game don't get me wrong, but not anywhere as good as people make it out to be. And the horrible voiceacting!!! >.> 

PS: I need to mention that my last sonic game prior to this was Sonic Advance, and I've missed out on the previous 3D sonic games

If you compare it to the BEST platforming games which is what you are doing, it does lack a bit.  It is a better game than Kirby's Epic Yarn...

The voice acting in Sonic games has always been bad, so what's your point?  It's actually improved a little bit.

Sonic Generations is better.