3rd party developers are immensly stubborn when it comes to Wii, particularly Japanese ones. I remember back in like 2007 Japanese developers were holding off on Wii developement because they were anticipating a Wii collapse and "waiting for a drastic change." Well lo and behold, that drastic change never came, so they continue to sit on the sidelines and wait for their savior the PS3 to make an impossible comeback, the comeback that will never happen.
PS3 is simply far more attractive to these developers, even if PS3 is performing poorly, its development costs are skyrocketing, and its software barely sells in Japan. This is because in releasing their games on a Sony platform, they don't have to worry about competing with Nintendo's games, which are generally superior. I suppose it's just easier to avoid Nintendo altogether rather than try to actually better their own games and outdo them.
Nintendo's greatest strength, the ability to make great and accessible games, is also their greatest curse, because it scares away 3rd parties that can't measure up.
