I've thought about this a lot, and the hypothesis I've come up with is that Nintendo's disruption is a greater threat to (some) third parties than it is to Sony and Microsoft. Or, at the very least, the threat is equal. Remember that third party demands have always provided Sony and Microsoft with a basic console strategy (perhaps MS more than Sony).
Taking this into consideration, we see that many third party publishers and developers were pushing for better graphics, high definition, and more complex controllers. These sustaining innovations support the business model of releasing "bigger better" sequels. So if Nintendo replaces the values of better graphics and complex gameplay, they will be disrupting the business models of many leading third parties. That is why, in my view, third parties have been reluctant to develop for the Wii in spite of its user base. Aiding in the Wii's success would be aiding in the disruption of their own business model.
Only smaller devs - who can ride the Wii upmarket, and more agile publishers like Sega, who can adapt to the new values - will aggressively develop for the Wii. All other publishers will avoid, or give poor support, to the platform. We saw a similar situation during the NES generation when the NES displaced the values of computer gaming. EA was not disrupted, but their values conflicted with Nintendo's, and they avoided the console for the entire generation.
Some third parties will never join Nintendo. But as Nintendo's internal games, as well as games from a select few third parties (Capcom, Sega, indy devs) move upstream - that is, move towards the hardcore - third parties will be attracted by the sales and will follow along. But Nintendo is starting from the bottom (non-gamers) up, so it will take time to work their way towards the most avid gamers.
I see.