I've asked the same question before...
here's a good explanation why:
Nobody has a set designed specifically for the NSR But If they did they would look like ducati or rc51 sliders. They have a rod that goes through the motor mounts and the puck bolt to there. Prolly the only way to make them strrong enough, but honestly the NSR motor mounts are pretty puny so any good hit with frame sliders attached would surely bend the frame.
http://www.nsr250.net/forums/viewtopic.php?t=2873&highlight=frame+sliders