In comparison to the PS3 and XBox 360 the Wii could have a lot more major third party exclusive games by the end of this generation than the other systems. Beyond the obvious argument about development cost and sales, consider how long these major third party exclusives take to develop for these systems ...
Metal Gear Solid 4 was said to have began development before the Tokoyo Game Show 2005, which means that it has already been in development for at least two and a half years (possibly closer to three); if Konami started Metal Gear Solid 5 the second they complete Metal Gear Solid 4, assuming a similar development cycle Metal Gear Solid 5 would be released in the dying days of the PS3. Metal Gear is not a unique situation as many of the PS3's biggest exclusive games have seen several delays, and games that were assumed to be released in 2007 won't be seen until late 2009.
The Wii's development lifecycle is much closer to that of previous generation systems, which for "Major" games is typically 18 to 24 months; the shorter lifecycle means that a developer who started work on the Wii today can still (probably) complete two games before the generation is (entirely) over.
