The fact of the matter was that sales dropped to the point where Ariens/Gravely could not afford to keep making them. In short you can't sell what people don't want. Would you spend $15,000 for a 20G? How about $7500 for a Pro12? That is what those tractors would sell for today. How did I get those figures? I went back to about 1972 and compared the price of a Gravely to the average price of a new car. I also took other price lists for later years as well.
In general the price of a new Gravely had not changed in over 30 years. A two wheel tractor cost about 25% of the cost of a new car. The top of the line 4 wheel tractors like the 816 and 20G cost about 50% of the price of a new car.