The problem with the A380 as a freighter is the Max Zero Fuel Weight is simply way too low. The 777-300 as a P2F conversion can carry as much as an A380 and the operating costs are far lower (and the 777F can carry even more). If you're willing to pay the operating costs for a quad, a 747 can carry much more than an A380.
Yes, the A380 can carry it a long way without refueling, but the world cargo market isn't set up that way - max sector length is around 4,000 miles. The A380 has a lot of volume, but there is little demand for high volume/low weight cargo aircraft.
Short story, the 747 was designed from day one to make a viable freighter - the A380 wasn't...