A lot of people all over the internet currently speculate that a lot of publishers and developers are moving onto the Wii, after all its selling at record speed, so its just good business sense to do so and soon the Wii would have plentyfull high quality third party tiles, right? Sadly for all those Wii owners, nope, that is not the case.
The Wii is, in terms of pure hardware power (RAM, CPU, GPU, etc.) a generation behind the rest, which means the Wii can't handle many games that are developed for the XBox360/PS3 right now, due to heavy use of crowd AI, physics or large environments. The result of this is that the Wii is an isolated platform, games might be developed for the XBox360 and PS3, but at the same time they can't be developed for the Wii. The word multiplatform this generation is getting the meaning of XBox360 and PS3, but leaves the Wii out in the dark.
So what does that mean for the Wii? It means that the Wii is up against the PS3 and XBox360, not up against the PS3 or the XBox360 when it comes to attracting third parties. So from a publishers standpoint that means you currently have 6 million Wii against 13 million PS3/XBox360. Its not rocket science to figure out for what platform a new game should be developed. Looking at the monthly sales numbers also turns out that the Wii is losing ground against the XBox360 and PS3, which means the potential for decent third party support is shrinking, not growing.
So while the Wii might sell more then XBox360 or PS3, it just isn't doing good enough to sell more then both combined and there lies the problem. Whenever the big potential AAA titles are searching for a platform, the XBox360/PS3 will be more attractive then the Wii. This doesn't mean the Wii won't get third party games, but it means that the Wii won't get the big titles. As a gamer, that doesn't look like a good future for the Wii, Nintendo themselves of course don't really have to care, they might still come out as number one this generation, but as you can see above, even if it becomes number one this still doesn't mean it will get good third party support.