Sega said no. That's it. I'm serious. They just suddenly went "no." to the Wii and didn't port it over. Here we go again, with Sega making the same bullshit mistakes. I'm dead serious. They didn't put it on the Wii because they didn't want to. I don't have a source, though.
Well in all honesty Sonic and the Wii have never really worked well together. Like every game that Sonic has been in for the Wii sucked. Like Sonic and the Secret Rings (Which is like the worst Sonic game I've ever played), Sonic the Black Knight (Which I've heard is a bad game in general), and I've heard that the Wii version of Sonic Unleashed was horrible, but the Xbox 360 version was better.
It's been known for quite a while now that Generations was not going to make an appearance on the Wii. Just as others have said, SEGA wanted to celebrate the 20th anniversary with an HD game, so the Wii is out of the equation. If things go well, it's quite possible to have a port done for the Wii U. Other than that if you think they're making mistakes you forget all the exclusive Sonic games that came out for the Wii.
On a side note Sonic Colors is one of the best things to happen to the Sonic universe and that was a pure Nintendo exclusive. Now it's time for the other platforms to have a turn.
I really find it funny how DYL seems to be the worst person for some how "not even having heard the most unbelievably vague hints of even the existence of games that everyone knows about", next thing, it'll be "I've never heard of Mario".