I think that the different control schemes are an issue too. The Wii Controller is totally different and you can't simply port games from PS3 to Wii... but you can simply port from Wii to PS3 (Aragorns Quest, Dead Space Extraction, etc.).
Nintendo could get into its own market by making exclusive control schemes. But at the same time they locked themselves out of easy ports. The combination of Graphics and Control scheme just make things difficult. You can say that the Wii as a single platform has a large userbase. But if you combine PS3, X360 and PC the situation just look different. That may be the reason why so many games are developped for those platforms. All 3 can offer a very similar experience and together they just offer the largest market, while the Wii is some market of its own due to the exclusive Controller and the different Graphics.
Another problem may be that a typical Wii Owner is expecting something different than a typical PS3 or 360 owner. Since Motion Control seems to be a strong selling Point of the Wii you have a separated user base. I bought the Wii mainly for retro gaming (i like the virtual console) and some Nintendo games... but I have a great problem with mandatory motion controls. So people like me, owning PS3 and Wii, would always take the PS3 Version over the Wii version because of optional motion gaming. In my opinion motion gaming is just its own genre... You just can't include it in every games and it doesn't add to every game but can instead lessen your experience considerably... But there are User who never had a console without motion controls before who could find games without motion controls simply lacking.
I think that Nintendo locked itself in its own market giving it some sort of monopoly over its competitors and they had a big success with this decision. But at the same time they locked themselves out of the "normal market" because the Wii is too different from the PS3, X360 and the PC.
