I'm sure the Wii couldn't handle certain games. Sonic Generations most likely wasn't one of them. There was a 3DS version after all. However, Sega did make Sonic Colors exclusive to Wii/DS the previous year so the absence of Sonic Generations probably had more to do with the lack of available teams. I suppose they could have farmed the game out to some random development team, but that certainly has its own set of cons.
It's hard to blame some of these companies for just not wanting to spend the resources on a Wii version of a game that may or may not sell. I get that, even if many companies left money on the table, but hindsight is 20/20 is and all that. Fortunately, Wii U won't have that problem for at least a few years because 3rd parties can just port games over, practically as is. Sure, they may not use the extra power, but I think fans should be happy to get the games as long as they aren't sloppy.