PS2 was the most popular system last gen. Going into this generation, everyone expected PS3 would be the most popular. But as XBOX360 came out first, people started developing on that first. PS3 and Wii came out at the same time and virtually little came out from third parties on either system as they waited to see who 'won'. But tons was made on the 360 still as most developers, both funded by MS and already in development, had a head start.
The only question you have to ask yourself is why after the first year of Nintendo selling out completely, and EPSECIALLY the second year when XBOX360 software sales for Japanese titles started to do 1/10th that of PS2 games, Third Parties didn't switch over to at least attempt making Wii titles beyond money makers and ports.
Six upcoming games you should look into:
