Piling on another point (mdasen is absolutely right about the route-model aspects, and the fact that megahub approaches "lost" to smaller planes flying routes to non-megahubs):
Airbus made one more big, big blunder with the A380: they deliberately and knowingly designed a jumbo that could never have a cargo variant or a cargo conversion. Although the 747's cargo conversion famously is due to Boeing thinking supersonic planes would take over the passenger market in the 70s (and thus the 747 needed a backup role it could take on once it had been displaced), Boeing stuck to the convertibility of the 747 through all those decades, and it's given them business both directly (through selling dedicated 747 freighters) and indirectly (through customers who bought used passenger 747s and wanted to convert, incurring all sorts of work on the airframes).
The A380 can never do this. This is not a "they don't want to" situation. This is a "the way they designed this plane, it can't be done".
First, the A380's middle-deck (the "ceiling" for the lower passenger level/"floor" of the upper level) is a structural component. You can't just yank it out of there and use the whole interior as a vast cargo space. It also cannot have a nose cargo door like a 747; the 747's upper-deck hump with elevated flight deck was created specifically to allow turning the nose cone into a cargo door, while the A380's flight deck is situated much lower to the ground for compatibility of training with the A330/A340.
Second, although the A380 is big, it's either not big enough, or not powerful enough. The increased interior volume over a 747 is large, but the increased capacity to carry weight... is not so large. In fact, an A380's payload capacity is lower than a 747's![1] Which means that if you solve the structural-deck issue, or work around it, you're getting a bunch of space that you can't use without going over the plane's weight limit.
[1] Technically, an A380 can take off and land heavier than a 747 (A380 max takeoff weight is 575,000kg, max landing weight is 394,000kg, compared to 447,700kg/346,091kg for a 747-8). However, an A380 is also heavier to begin with (by about 80,000kg), and must carry more fuel. The maximum payload for an A380 is thus about 84,000kg, compared to 132,630kg in a 747-8 freighter.