By the late 1970's, 100's were not a popular class, and many tracks did not race them, even though the bikes were fun to ride.
I think as the 80cc bikes got taller and faster, there was less of a need for a transition class between minicycles and full race 125's. And the later 'full race' 100s like the YZ and RM did not have much low end power, so they did not make very good trail bikes, which also hurt sales.
Yamaha used to make a YZ175 for a couple of years as well.