Nic,
When we had the "Quietline Run Difference" discussion a few weeks back, we had talked about the cast iron to cast aluminum changeover on the grill casting, and also the rear axle "bugles" (as I call them). Our consensus was cost. Aluminum is cheaper to cast, and is a less expensive alloy. Also, it is lighter, so each Cub weighed less. If I remember right, in the spec sheets between an early (1975ish) vs a late (1979ish) Quietline, there was 70 or 80 pounds difference in the spec'd curb weight. In the grand scheme of things, if IH could shave 50 or 60 pounds off of each of the thousands of Cub Cadets they shipped, it would cost them less money, because as we all know, transportation companies charge you by the pound. Then, look at the next generation, the 82 series. They were completely redesigned again, had less body parts, I can't confirm it, but I think the sheet metal they made the hood and side panels with was thinner than the earlier Cubs. They eliminated the huge castings for the grill, with a single, lighter grill housing. They were looking to save money, and turn a bigger profit, unfortunately a quick and easy way to make something cheaper is use less expensive stuff to build it........
I know it is apples to oranges, but look at all the IH Cubs, they were 600-800+ lbs, the new MTD CC's are what 300-400 lbs?
EDIT: OOPS, I misread your original question, I thought it said why, not when...... sorry.