AS much as I admire the airplane....It’s just too expensive to operate,
(4 engines 6 Landing Gears 22 wheels 20 brakes etc)
Compare that to the 777-300 (2 Engines, 3 Landing Gears, 14 wheels, 12 brakes etc)...Maintaining Airplanes are very expensive and airlines (Especially in the US) prefer twin engine birds.
The new 747-8 is having the same problem again due to maintenance cost.