[jakub_g]

Apparently it's about the weight/space ratio which makes is not economical to fly cargo with A380:

"An A380-F would be too fat to fly at a profit: The plane would hit the maximum payload (a constraint of weight) before its maximum cubic space (a constraint of volume). Its design can’t support the maximum payload required to generate a profit.

(...)

The A380-F would be able to carry 60% more volume than the 747, but only 28% more weight. It wouldn’t be fully loaded at typical levels of air cargo density, or at least nothing close to what can be supported by the thrust capacity of the 747."

https://www.flexport.com/blog/airbus-a380-no-cargo-equivalen...