The A380 doesn't pencil out to be a good freighter - lots of volume, but it can't carry much weight. Short story is the max zero fuel weight is simply too low. As a result - depending on how much weight they can take out vs. how much structure they need to add in - an A380 Freighter conversion would be limited to around 100 tons of cargo.
To put that into perspective, a 777 Freighter can carry over 100 tons, and burns a lot less fuel in the process.
Combined with the difficulty of loading/unloading the upper deck, an A380 freighter conversion simply doesn't justify the costs.
Converted 777s and 747s make far better economic sense.