Because Nintendo has taken a huge risk and a whole new direction with these controllers. Remember how much the MS Sidewinder gamepad with accelerometers bombed when it launched? When the Wii controller was first shown, it was scorned to hell (as much as PS3's "boomerang"). It took a while to get accepted.
Now that it is out and has proven itself that it works, Sony and Microsoft might take more notice of the new controller. Sony has already done it in part with PS3's sixaxis. The latter is being accused of ripping Nintendo, but AFAIK gamepads with accelerometers were done first by... Microsoft.
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the ps3 and xbox cant just copy it. if they did, it wouldnt work with any of the current games, and all subsequent games would have to be programmed for both a regular controller and the motion ones. it wouldnt work.
all of wii's games will be made specifically for the new controllers. you should get control of your games unlike anything we have experienced as of yet. i have a feeling ps3's sisaxis or whatever simply wont be implemented in most games. it will be useless.