I don't buy that there are no complimentary upgrades because this is a high-demand F route. If that were the case, they'd offer up the upgrades and they would just be rare. They wouldn't lose much.

My theory is that they know it is a long painful flight in Y, particularly from ATL, and they try to get more people to purchase F by not allowing upgrades. So they are trying to create more demand by denying complimentary upgrades.