This has clearly been the case since the demise of the DC. I think after that, a lot of old Sega fans kind of migrated to the Gamecube, and Sega did put out a lot of GC games, including Sonic, Billy Hatcher, a port of Skies of Arcadia, Amazing Island, Virtua Quest, Phantasy Star Online 1 & 2, Super Monkey Ball, etc.
They tried to spread the love around, putting Shinobi on PS2, and Panzer Dragoon and Toejam & Earl on Xbox, but sadly all of those games sold like shit. I honestly think that if they had put them all on the GC, they would have sold well, since like I said, a lot of Sega gamers went to their old rival Nintendo.
As far as Sonic Generations goes specifically, a lot of people were excited because it was a solid looking Sonic game with old school style side scrolling levels. The 3D levels still kind of played like shit. But had they ported Generations to Wii, again, I think it still would have sold really well.
I'm just curious to see what their third Wii U exclusive is. I really wish the rumor of a Sonic/Mario or even Nintendo/Sega racing game were true. That could be amazing if done right.