Okami wasn't very well known, due to some issues between the company that made it, Clover, and the company that owned them, Capcom. The game barely made press over here when it was released on PS2, and even less so when they made a Wii version. After the Okami 'failed' (it was an amazing game), Clover shut down and Capcom took over the sequel. Because of that, since Capcom is fairly big, Okamiden got a little more advertisement that it's predecessor, but the series itself just isn't really heard of. It is a shame, because they are wonderful games with a creative style and myth-filled story.
Then I wonder how they do their metrics. Bad metrics keep killing good tv series. Remember 1960's Star Trek being canceled? A goof-up by Nielsen ratings. Maybe it's the same with video games.