Because this creates a problem with production and essentially doubles the complexity of the supply chain. AMD and NVIDIA seem to be simplifying their VRAM chip orders to 2GB chips only. Adding 1GB chips into the mix will cut into the 2GB chip supply in some form or fashion. Not to mention if the lower configuration doesn't pan out for some reason, you're now left with stock that you can't use. Sure you could nerf the 2GB chips to 1GB, but... why not just use 2GB? And while I can't say this is a thing for memory, I'm sure the yield rate for 2GB chips is high enough now that there's no point in trying to make 1GB chips out of the defects.