Nintendo is a very cautious fiscally conservative company, even now in the face of incredible success. They recently lowered their forecasts for the year in light of stagnate Japanese sales so avoid possibly disappointing stockholders. Integrating motion plus into the current Wii Remote design would likely some redesigns, then setting up manufacturing lines to handle the change, and all that costs money.
If they were to integrate M+ (I personally don't think they will.) it probably wouldn't be until late in the generation. As of right now, they don't know how big M+ will end up being, how well received it will be, how many 3rd party developers will actively support it. Also if they earn more money selling it separately, there'd be even less incentive to integrate it.
If M+ ends up being heavily supported amongst a great many popular titles to a point where consumers expect it with the system, I could see them doing it then. Especially if Wii sales have finally stabilized in North America by then. But I think it'd be more likely to just bundle M+ with the Wii and Wii remotes since it's a fairly small and likely unobtrusive item and save the redesign for their next console's controller.







