More kids need to realize that MotionPlus was made by a different company and that the technology didn't exist(and in a way still doesn't) until recently.
You're half right. The tech used is simply a gyroscope (solid state MEMS, not the mechanical spinning top kind), which has been around for a while (r/c helicopters have been using these for years for heading hold for example). However, the tech really hasn't existed in a small enough form factor or has been energy efficient enough (not to mention cost effective enough, the gyro's you buy for r/c heli's are definitely not cheap and cost much more than a Wii Remote) to build into a video game controller, which are the real reasons this was not built in from the start. I can't be arsed to find a quote for this, but a couple of months after they showed the M+ at E3 last year, it was mentioned that they were in fact still working on getting it's power usage down to a reasonable level (since it feeds off the Wii Remote batteries).