There is a huge advantage to the 360's market share that partially explains its great software sales. It has been out, and the higher quality 3rd party software released for it has been well spaced out. There has been a pretty steady stream of high quality 3rd party software for the 360 since it was about 10 months old, and most titles have had a lot of hype and time to sell before the next was released.
Part of the problem with the Wii is that there was a huge drought of good 3rd party stuff, then a huge influx. And now people are judging the 3rd party sales at a time when Nintendo just released a very high quality game (Metroid Prime 3) and is on the verge of releasing possibly its best game in a decade (Super Mario Galaxy). When Halo 3 was released, all 3rd party software on the 360 underperformed. SMG will likely have the same affect, making November a *very bad* month to release SMG.
At this point, it's already clear that 3rd party games can do quite well on the Wii. Better than the PS3, anyhow. Understandably, Eidos wants Nintendo to do everything they can to encourage success for 3rd party games (ensuring enough production of discs, etc) during the holiday season because it will have a big effect on their bottom line. As much as he wants to make people (especially Nintendo) think that they could ruin their empire this holiday if Eidos games don't sell well enough, the simple fact is that Nintendo has already entrenched itself well enough to earn a lot of good third party support coming up.
Right now, it's saturated, however.
