I think 3rd parties are realizing there are greener pastures on Nintendo's console right now. Why would they support a bunch of "hardcore" PS3/360 owners if they can only buy 14 million consoles total? The Wii has sold 10 million so far, and is on track to outsell the 360 + PS3 put together.
So, if each game on the Wii is cheaper to produce, and there is a bigger audience on the Wii, where will you develop your games? (Think PS1)