We live in a world of limited resources, and the videogame market is no different ...
There is limited third party development resources, and every third party developer who begins an exclusive Wii game can not devote those resources towards a PS3 or XBox 360 game. As the will continues to outsell the PS3 and XBox 360 by a wide margin more and more publishers and developers will be devoting resources towards the Wii to produce Wii games which maximize the capabilities of the Wii (therefore making them exclusive games).
There is a limited number of consoles that will be bought by consumers. Most people who play videogames do not see it as their primary recreational activity (and it may not be that important to them in general); these people tend not to buy a new console as long as their current console has enough new content for them to remain entertained by the system.
