Nintendo's Shigeru Miyamoto has stated that the inclusion of Wii Motion Plus technology into Wii Remotes was simply not feasible at the launch of the Wii.
Speaking to our broadsheet newspaper The Telegraph:
"It's a continuous evolution. Not all the Wii software requires MotionPlus, so it is appropriate for us to position it as a peripheral."
"Also, it was not technically feasible for the technology to be realized with such a compact size. The fact we are now being able to sell Motion Plus at this price, at this size, is the result of technological advancements in the field of microelectro-mechanical systems, which took place after Wii launched."