Why indeed.Hmmm.
Dirt track number 1 plate awards stop in 2015. Speedway stops in 2015. Roadracing stops in 1995 (!) and that's after a gap from 1990-1993. Mini road racing appears to continue (we will grant them 2020 due to other circumstances) - although three riders having taken multiple number 1 plates over a period of only 9 years makes me question how many riders are actually involved. Motocross stops in 1999. Enduro stops in 2009. Cross-country (whatever that is) stops in 2002. Hare scrambles / cross country appears to continue. Trials is current. Ice racing is current. There are several others that stop varying degrees of a long time ago.
Do you see the pattern of the CMA slowly giving up? Or, perhaps more accurately, other disciplines slowly giving up on the CMA.
There's more to motorcycling than just trials, hare scrambles, and ice racing. It's not like dirt track, speedway, roadracing, and motocross haven't continued. They're just continuing without being associated with the CMA/FIM.
Why?
Back in "the day" there were provincials (perhaps regionals too) and nationals and club races.
Now, all the "national" level events are all held in the same place with the same riders. In my opinion that number 1 plate is essentially a club level award. A skilled rider wins it, much more-so than I, but in such limited competition as to make it near meaningless.
Back then it was communicated to me that the "leadership" (I threw up a little using that word here) of the organization was temperamental. If you got on their bad side then that's were you would forever remain, so act accordingly. You weren't going to get any answers and the people who MIGHT weren't going to take the chance of asking the questions.
Back to your question. Back in the 90's & early 2k's the competitors, the event organizers and sponsors wanted more from the events. The CMA had become a thread-bare organization and highly centralized around a few people. They didn't want to change and arguably couldn't meet the expectations at the time. They certainly gave the appearance that they couldn't be bothered. So the sport/pastime splintered into a collection of organizations focused on specific disciplines, with the CMA obstinately hanging on to their FIM issued gavel/hammer.
That's just the way I saw it.
Incidentally, I don't know the difference between a cross-country or a hare scramble either, but I rode both. Seems to me the hare scramble courses were shorter, but really the only thing I remember is that it was all good fun.