In a lot of ways Nintendo follows an "Old Fashioned" business model (because they're a very old company) where they do their business in cash rather than credit; and the current credit crisis demonstrates why this is/(could be) a wise approach. With this in mind, Nintendo has to produce a product which sells well at a price where they're not losing a lot of money on it; on top of this they're also very aware on what a consumer is willing to pay for a particular product.
I suspect that, when the Wii was launched, using the technology inside of the Wii MotionPlus in the standard Wiimote would have pushed the manufacturing cost of the Wiimote to a level where it wasn't profitable at $40 (and the nunchuck couldn't be profitable at $20) and they knew that few people would be willing to spend $50+ on a Wiimote (or $30+ on a Nunchuck) ...
