It shouldn't need more explanation, but maybe I have to explain for you.
Based on the current info/rumors the price of 6970/50-1GBs in CFX would be ~30-40% less than 6970/50-2GBs, and probably eve less for GTX560 SLI. Do you think people are fool to pay that much extra to get 2GBs? You see, the price plays a big role, specially when you have to pay for 2 cards!
AMD's 1GB-move has already killed those 2GBs, and 560 will come on top of that and make sure that nobody buys those 2GBs for almost double-price, ruffly.