Just the signs of time, there being replaced by the Acadia's, Terrain's and Canyon's for the compact size SUV's and the full size line would be the Sierra and Yukon's. With all these newer SUV's and the plant that make them were retro fitted for them, the older SUV's which had a different platform productions are no longer being made.
If GM or the other manufactures never change there product line and kept the same platform of production, we would still be riding on a horse and buggy.