Cause it makes no more sense than comparing it with the MD11. As for Thai and their relationship with Southern and the B777, I have no idea why Thai pulled out. I know why Air France tried to ditch theirs, and that was poor homework. What I do know is that FedEx cannot get enough of them, and that the experience AeroLogic has had with theirs was, at least partly, the reason why LH Cargo went and bought a few. I also know that the aircraft actually does more than the original brochure numbers, which is quite a surprise indeed. Later iterations have seen further improvements, and it is not unlikely we will see 108 tons payload in the future.
Why does a 777/MD11 make more sense? The MD11 doesn't have the capacity of the 777, and probably can't come close to the range of a 777 with a lighter load...
FedEx hauls mainly small packages, not heavy freight. AeroLogic does the same for DHL. I don't know what LH's plans are.
The package and freight markets are significantly different. As others have already pointed out, there is significant advantage with the 744F for heavy/outsize freight. The 744 can carry 108T HKG-ANC now, and lighter loads to 7600+ NM (e.g., 45T HKG-CVG).