It is all about fuel burn per seat mile. Click on the link to see the stats. https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft#Long-haul_flights
The A380 had inferior fuel burn to the 747, the 777 and it is not even close to the 787. In addition if you can not fill that monster up with passengers its fuel burn per seat mile is obscene.
The A380 is a technological marvel. It is an economic disaster due to a state run business designing airplanes instead of market forces.
One of the big failures of 380 is the short-sighted planning. While it CAN likely be adjusted to cargo, it will not be very efficient at it, what with a single possible loading entrance.
The 747 was built with the cab at the top deck - which allows easy entrance from the front of the plane as well as the rear.
This has made it hugely popular for freighter companies, and with the take-off of UPS and FedEx and then amazon, Boeing has it in the bag.
Airbus will never be able to really take advantage of the space in the 380 with loading encumbered.
Indeed, America designs systems, not just the crafts