People also forget that Nintendo never runs their hardware as a loss-leader, which means that they are making money just by selling the base system; this is not true with Xbox and PS systems, which rely on the per-unit moneys from software sales. So with the gazillion Wii units out there, Nintendo is happy, sure, but they were also still making money with each Gamecube sold.
And has been pointed out here more eloquently than I will manage: they are sufficiently served by their own IP, and don't need 3rd party stuff to further their control of their particular market.
What's sad is that the Wii has such a unique control system that it's very hard to port anything to it, as almost every publisher learned this generation. And the customers for the most part expect perfect ports, as I can attest from people who write my company all the time wondering why the Wii version doesn't have as much content, doesn't look as good, and doesn't support all the feature of the 360 and PS3 version of our flagship title.
So anything made for the system really needs to be a unique title, developed solely for Wii, and as we can see from Dead Space Rail Shooter, that's not a safe proposition.
