It is because the very large difference in fuel weight for a flight leaves radically increased capacity for carrying paying freight. As LH2 produced on-site at airports gets cheaper than Jet-A fuel, the gap widens.
Volume is cheap, on aircraft. If a normal freighter today takes off with 25t of kerosene, the similar-sized LH2 craft needs only 10t of LH2. That means it can take an additional 15t of paying cargo.