Yamaha also did the NS (or was it MS?) 10s. I have a pair of their 8 inch monitors.
No idea why they stopped making them, tho there was a consumer scare about the beryllium used in the tweeters of the models, too. The big ones were supposed to be right up there in the pro world, tho I never heard them.
I think Middleman is right - there were enough problems associated with them that the small market and margins didn't warrent continued production. Finding new cones and tweeter material might have been "cost prohibitive." I'll say one thing for Yamaha, they do stuff right. My speakers and amp still work 30+ years later. Maybe making stuff right doesn't pay off.
@