The Wii was probably a lost cause for "mature" titles within its first year. Many were starting to see the motion controls were not adding much to the immersion. The 360 and PS3 had plenty of what older players wanted, and many likely didn't have extra cash for another system. Even good kid friendly third party games were lost under the sea of shovelware the Wii shelves collected. It had a few non-Nintendo gems that didn't have the look of a kid's game, but not enough at once to really get noticed by the crowd.
Okami came out at a bad time and probably could have used a better ad campaign. The PS3 (and Wii) was looming over the horizon. A few of my friends dismissed it as "some anime game." The only way I heard about it was the Game Informer review that showered it with praise. The Wii port didn't do well because of reasons I already stated (and it was the only game where the Wii's IR pointer controls felt like a downgrade to me). I wonder if the HD remaster met sales expectations? The DS sequel did better financially, but not by much. Now, if only we could get one last sequel... (Wii U gamepad controls would be great but that system is a curse to good third party games.)
To add another forgotten critically acclaimed, poor selling franchise, Klonoa sure got beat down a lot. Seeing some let's plays of the 2 (er, 3 with the original's remake) console titles had me regretting not trying them out as a kid (not enough money/present allotment). If Namco could make a good compilation, I'd pick them up fast.