Re: It wasn't cancelled because it was expensive to operate
The single-deck jets are already a known quantity. Boeing and various other third party companies already know what is required to turn a single deck passenger jet into a freighter. From the A320/B737 up to the A330/B777 there's plenty to pick from.
The A380 will be more difficult to convert, mostly because Airbus canned the A380F project (which cost them a massive FedEx order), and there'll have to be research into how much the intermediate floor (the floor between cargo and the 'main' deck) needs to be strengthened to allow containers on the main deck, and also what needs to be done for a cargo door on the main deck.
The upper floor would also need strengthening, but probably not quite to the extent the main floor would need to be. I think FedEx had meant to put the voluminous but light paletted cargo on the upper deck, with containers in cargo and on the main floor, but who knows. The B747F doesn't run cargo on the upper deck, so it's effectively the same as a single-decker. :-)